mkdir: quote invalid modes in diagnostics

[0Whisperos]

Fixes #13834.

mkdir -m now handles invalid modes consistently across stderr environments:

• When stderr is not a terminal (for example, in scripts or pipelines), it reports a GNU-compatible one-line diagnostic such as invalid mode 'a=\001'.
• When stderr is a terminal, it preserves the rich mode diagnostic introduced in mkdir: point at the failing part of an invalid mode #13826 while escaping control characters in both the error message and the rendered command snippet. The highlighted span is adjusted after escaping.

This keeps non-printable mode characters visible instead of emitting them directly to stderr.

Tests cover both piped and terminal stderr for a=\x01.

[github-actions]

Binary size comparison:

Individual binary size comparison VS main (threshold: >=5% AND >=4 KB).

Total size of compared binaries: 149.13 MB (+524 KB, +0.34%)

Significant per-binary changes:
  mkdir       1.09 MB ->    1.22 MB  (+128 KB, +11.43%)
  install     1.24 MB ->    1.36 MB  (+116 KB, +9.12%)
  mkfifo      1.09 MB ->    1.20 MB  (+116 KB, +10.43%)
  mknod       1.09 MB ->    1.21 MB  (+116 KB, +10.36%)

[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/tail/tail-n0f (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/csplit/csplit-heap is now being skipped but was previously passing.