fsx,fsxc: fix PrintUsage() to identify program

Partially addresses this bug[1], but only partially because it
still happens in Unix (when not installed via `dotnet
tool install`), due to the fact that in Linux
and macOS we still use launcher.sh instead of fsx proj.

[1] #34

fsx/Program.fs

@@ -141,7 +141,7 @@ let fsxcMainArguments =
         Seq.append fsxcArgs (Seq.singleton userScriptPath) |> Seq.toArray
     | None -> fsxcArgs |> Seq.toArray
 
-Program.Main fsxcMainArguments |> ignore
+Program.OuterMain fsxcMainArguments |> ignore
 
 match maybeUserScriptPath with
 | None ->

fsxc/Fsxc.fs

@@ -40,6 +40,10 @@ type BuildResult =
     | Failure of BinFolder
     | Success of ExeTarget
 
+type ProgramInvocationType =
+    | FsxLauncherScript
+    | FsxcPureInvocation
+
 exception NoScriptProvided
 
 module Program =
@@ -56,9 +60,18 @@ module Program =
              tmpNuget)
 #endif
 
-    let PrintUsage() =
+    let PrintUsage(invocationType: ProgramInvocationType) =
+        let programInvocation =
+            match invocationType with
+            | FsxcPureInvocation -> "dotnet fsxc"
+            | FsxLauncherScript -> "fsx"
+
         Console.WriteLine()
-        Console.WriteLine "Usage: dotnet fsxc [OPTION] yourscript.fsx"
+
+        Console.WriteLine(
+            sprintf "Usage: %s [OPTION] yourscript.fsx" programInvocation
+        )
+
         Console.WriteLine()
         Console.WriteLine "Options"
 
@@ -890,14 +903,17 @@ let fsi = { CommandLineArgs = System.Environment.GetCommandLineArgs() }
 
             exeTarget.Exe
 
-    let private InnerMain(argv: array<string>) =
+    let private InnerMain
+        (invocationType: ProgramInvocationType)
+        (argv: array<string>)
+        =
         if argv.Length = 0 then
             Console.Error.WriteLine "Please pass the .fsx script as an argument"
-            PrintUsage()
+            PrintUsage invocationType
             Environment.Exit 1
 
         if argv.Length = 1 && argv.[0] = "--help" then
-            PrintUsage()
+            PrintUsage invocationType
             Environment.Exit 0
 
         let parsedArgs =
@@ -942,13 +958,19 @@ let fsi = { CommandLineArgs = System.Environment.GetCommandLineArgs() }
 
         0 // return an integer exit code
 
-    let Main argv =
+    let private WrapperForMain invocationType argv =
         try
-            InnerMain argv
+            InnerMain invocationType argv
         finally
 #if LEGACY_FRAMEWORK
             if nugetExeTmpLocation.IsValueCreated then
                 nugetExeTmpLocation.Value.Delete()
 #else
             ()
 #endif
+
+    let internal Main argv =
+        WrapperForMain ProgramInvocationType.FsxcPureInvocation argv
+
+    let OuterMain argv =
+        WrapperForMain ProgramInvocationType.FsxLauncherScript argv