tail: fix panic on header write error

[gabrielhnf]

Fixes #11993

Problem

When running tail on multiple files and redirecting output to a full device (e.g. /dev/full), uutils panics instead of exiting gracefully:

thread 'main' panicked at …/std/src/io/stdio.rs:1165:9:
failed printing to stdout: No space left on device (os error 28)

Instead of the correct GNU behavior:

tail: write error: No space left on device

Cause

HeaderPrinter::print used println! to write the ==> file <== headers to stdout. println! panics on write failure rather than returning a Result, so any write error (such as ENOSPC from /dev/full) caused an unhandled panic instead of a clean error message.

Fix

Replaced println! in HeaderPrinter with write_all and flush on a locked stdout, both of which return io::Result. The methods now return UResult<()> and propagate write errors upstream instead of panicking.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/resolution (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/symlink (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.
Congrats! The gnu test tests/printf/printf-surprise is now passing!