unix: change Setrlimit/Prlimit to always call syscall functions

As of Go 1.21 syscall.Setrlimit and syscall.prlimit can affect
starting a new process, by restoring the original NOFILE rlimit.
That is recorded locally in the syscall package, so just always
call the syscall functions.

For golang/go#46279

Change-Id: I2f3dafe5562a7dde1297bad6f5d34a80af5d620b
Reviewed-on: https://go-review.googlesource.com/c/sys/+/476695
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

unix/syscall_unix.go

@@ -587,3 +587,10 @@ func emptyIovecs(iov []Iovec) bool {
 	}
 	return true
 }
+
+// Setrlimit sets a resource limit.
+func Setrlimit(resource int, rlim *Rlimit) error {
+	// Just call the syscall version, because as of Go 1.21
+	// it will affect starting a new process.
+	return syscall.Setrlimit(resource, (*syscall.Rlimit)(rlim))
+}