runsc: Fix newline for cmd usage display

## Summary
Originally, some of the commands' usage display didn't achieved to show newline, while some of them does, which are more user-friendly, and won't confuse the message if there're more which should be in the nextline.

Fix this issue by returning usage string with a newline.

## Tests
Before the change some commands display their usage like the following, for example `runsc start`
```
vax-r@vaxr-ASUSPRO-D840MB-M840MB:~/gvisor$ runsc start
start <container id> - start a secure container.vax-r@vaxr-ASUSPRO-D840MB-M840MB:~/gvisor$
```
Or `runsc wait`
```
vax-r@vaxr-ASUSPRO-D840MB-M840MB:~/gvisor$ runsc wait
wait [flags] <container id>  -checkpoint
    	wait for the next checkpoint to complete
  -pid int
    	select a PID in the container's PID namespace to wait on instead of the container's root process (default -1)
  -rootpid int
    	select a PID in the sandbox root PID namespace to wait on instead of the container's root process (default -1)
```
We can see in the case of `runsc wait`, the flag `-checkpoint` is expected to be shown in a way which is aligned with other flags, like
```
vax-r@vaxr-ASUSPRO-D840MB-M840MB:~/gvisor$ runsc wait
wait [flags] <container id>
  -checkpoint
    	wait for the next checkpoint to complete
  -pid int
    	select a PID in the container's PID namespace to wait on instead of the container's root process (default -1)
  -rootpid int
    	select a PID in the sandbox root PID namespace to wait on instead of the container's root process (default -1)
```

After the change , we can see those commands with this issue are being fixed.
```
vax-r@vaxr-ASUSPRO-D840MB-M840MB:~/gvisor$ runsc start
start <container id> - start a secure container.
vax-r@vaxr-ASUSPRO-D840MB-M840MB:~/gvisor$
```
```
vax-r@vaxr-ASUSPRO-D840MB-M840MB:~/gvisor$ runsc wait
wait [flags] <container id>
  -checkpoint
    	wait for the next checkpoint to complete
  -pid int
    	select a PID in the container's PID namespace to wait on instead of the container's root process (default -1)
  -rootpid int
    	select a PID in the sandbox root PID namespace to wait on instead of the container's root process (default -1)
```

FUTURE_COPYBARA_INTEGRATE_REVIEW=#11324 from vax-r:fix_usage_display 83f9f18
PiperOrigin-RevId: 710756149

Author: vax-r

runsc/cmd/boot.go

@@ -201,7 +201,7 @@ func (*Boot) Synopsis() string {
 
 // Usage implements subcommands.Command.Usage.
 func (*Boot) Usage() string {
-	return `boot [flags] <container id>`
+	return "boot [flags] <container id>\n"
 }
 
 // SetFlags implements subcommands.Command.SetFlags.

runsc/cmd/checkpoint.go

@@ -55,8 +55,7 @@ func (*Checkpoint) Synopsis() string {
 
 // Usage implements subcommands.Command.Usage.
 func (*Checkpoint) Usage() string {
-	return `checkpoint [flags] <container id> - save current state of container.
-`
+	return "checkpoint [flags] <container id> - save current state of container.\n"
 }
 
 // SetFlags implements subcommands.Command.SetFlags.

runsc/cmd/create.go

@@ -62,8 +62,7 @@ func (*Create) Synopsis() string {
 
 // Usage implements subcommands.Command.Usage.
 func (*Create) Usage() string {
-	return `create [flags] <container id> - create a secure container
-`
+	return "create [flags] <container id> - create a secure container\n"
 }
 
 // SetFlags implements subcommands.Command.SetFlags.

runsc/cmd/debug.go

@@ -64,7 +64,7 @@ func (*Debug) Synopsis() string {
 
 // Usage implements subcommands.Command.
 func (*Debug) Usage() string {
-	return `debug [flags] <container id>`
+	return "debug [flags] <container id>\n"
 }
 
 // SetFlags implements subcommands.Command.

runsc/cmd/delete.go

@@ -45,7 +45,7 @@ func (*Delete) Synopsis() string {
 
 // Usage implements subcommands.Command.Usage.
 func (*Delete) Usage() string {
-	return `delete [flags] <container ids>`
+	return "delete [flags] <container ids>\n"
 }
 
 // SetFlags implements subcommands.Command.SetFlags.

runsc/cmd/gofer.go

@@ -112,7 +112,7 @@ func (g *Gofer) Synopsis() string {
 
 // Usage implements subcommands.Command.
 func (*Gofer) Usage() string {
-	return `gofer [flags]`
+	return "gofer [flags]\n"
 }
 
 // SetFlags implements subcommands.Command.

runsc/cmd/install.go

@@ -52,8 +52,7 @@ func (*Install) Synopsis() string {
 
 // Usage implements subcommands.Command.Usage.
 func (*Install) Usage() string {
-	return `install [--runtime=<name>] [flags] [-- [args...]] -- if provided, args are passed to the runtime
-`
+	return "install [--runtime=<name>] [flags] [-- [args...]] -- if provided, args are passed to the runtime\n"
 }
 
 // SetFlags implements subcommands.Command.SetFlags.
@@ -212,8 +211,7 @@ func (*Uninstall) Synopsis() string {
 
 // Usage implements subcommands.Command.Usage.
 func (*Uninstall) Usage() string {
-	return `uninstall [flags] <name>
-`
+	return "uninstall [flags] <name>\n"
 }
 
 // SetFlags implements subcommands.Command.SetFlags.

runsc/cmd/kill.go

@@ -46,7 +46,7 @@ func (*Kill) Synopsis() string {
 
 // Usage implements subcommands.Command.Usage.
 func (*Kill) Usage() string {
-	return `kill <container id> [signal]`
+	return "kill <container id> [signal]\n"
 }
 
 // SetFlags implements subcommands.Command.SetFlags.

runsc/cmd/list.go

@@ -51,7 +51,7 @@ func (*List) Synopsis() string {
 
 // Usage implements subcommands.Command.Usage.
 func (*List) Usage() string {
-	return `list [flags]`
+	return "list [flags]\n"
 }
 
 // SetFlags implements subcommands.Command.SetFlags.

runsc/cmd/metric_export.go

@@ -47,8 +47,7 @@ func (*MetricExport) Synopsis() string {
 
 // Usage implements subcommands.Command.Usage.
 func (*MetricExport) Usage() string {
-	return `export-metrics [-exporter-prefix=<runsc_>] <container id> - prints sandbox metric data in Prometheus metric format
-`
+	return "export-metrics [-exporter-prefix=<runsc_>] <container id> - prints sandbox metric data in Prometheus metric format\n"
 }
 
 // SetFlags implements subcommands.Command.SetFlags.