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 io::Result and propagate write errors upstream instead of panicking.