prog: support optional resources

[a-nogikh]

There are cases when the ability to generate a system call that produces a resource value does not indicate that we should disable all dependent system calls.

Add support for resource N[T, opt] resources. This could be useful at least for #4565 and hopefully resolve many situations that led to #2721.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.2%. Comparing base (e104824) to head (47beee0).

Additional details and impacted files

Files	Coverage Δ
pkg/ast/ast.go	71.4% <ø> (ø)
pkg/ast/clone.go	100.0% <100.0%> (ø)
pkg/ast/format.go	95.1% <100.0%> (+0.1%)	⬆️
pkg/ast/parser.go	90.6% <100.0%> (+0.1%)	⬆️
pkg/ast/walk.go	100.0% <100.0%> (ø)
pkg/compiler/check.go	95.6% <100.0%> (+<0.1%)	⬆️
pkg/compiler/compiler.go	96.8% <100.0%> (-<0.1%)	⬇️
pkg/compiler/gen.go	93.7% <100.0%> (+<0.1%)	⬆️
prog/resources.go	95.9% <100.0%> (ø)
prog/types.go	73.7% <ø> (ø)

... and 4 files with indirect coverage changes

[a-nogikh]

From an offline discussion with @dvyukov:

We can already do

struct {
 f    dm_ctl_dev[opt]
}

But the problem is that we get the following error afterwards:

sys/linux/dev_dm_ctl.txt:9:1: resource dm_ctl_dev can't be created (never mentioned as a syscall return value or output argument/field)