runcon: report a stdout write error instead of panicking

[leeewee]

Fixes #13379

runcon with no arguments prints the current SELinux context with println!, which aborts (exit 134) when the stdout write fails — e.g. output redirected to a full device. GNU reports the write error and exits 1.

print_current_context now writes to a locked stdout and propagates the io::Error as a new Error::Write, so a write failure is reported as write error: <reason> (exit 1), matching GNU.

Before:

$ runcon > /dev/full
thread 'main' panicked at library/std/src/io/stdio.rs:...:
failed printing to stdout: No space left on device (os error 28)
Aborted (core dumped)          # exit 134

After:

$ runcon > /dev/full
runcon: write error: No space left on device        # exit 1

This matches GNU byte-for-byte (/usr/bin/runcon: write error: No space left on device, exit 1) and is the same write-error class as chcon (#13061) and kill (#13297). A regression test is added (/dev/full, Linux-gated).

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/misc/tty-eof (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/env/env-signal-handler was skipped on 'main' but is now failing.

[xtqqczze]

Suggested change

	#[error("{}", translate!("runcon-error-write-error", "error" => .0))]
	#[error("{}", translate!("runcon-error-write", "error" => strip_errno(.0)))]
	Write(io::Error),

[leeewee]

Applied, thanks!