tac: avoid mmap on regular files to prevent SIGBUS on truncation

[mango766]

Summary

• Replace mmap-based file reading in tac with read_to_end() to prevent SIGBUS crashes when input files are truncated during read
• Remove the now-unused try_mmap_file() function
• Retain mmap for stdin (via temp files owned by the process) where external truncation is not a concern

Context

When tac memory-maps a regular file and another process truncates that file concurrently, accessing the now-invalid mapped memory region triggers SIGBUS and crashes the process. This is a real concern in log rotation scenarios where tac might be reading logs that get rotated/truncated at the same time.

GNU tac avoids this by not directly memory-mapping untrusted input files. This PR brings the same safety to the uutils implementation by reading files into a heap buffer instead.

Test plan

• All existing tac tests should continue to pass (file reads, stdin, separators, regex modes)
• The reproduction scenario from the issue should no longer crash:

  dd if=/dev/zero of=/tmp/tactest bs=1M count=10 2>/dev/null
  (sleep 0.001; truncate -s 0 /tmp/tactest) &
  tac /tmp/tactest

Fixes #9748

Generated with Claude Code

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/unexpand/bounded-memory is now being skipped but was previously passing.

[sylvestre]

please add a test