chmod,mkdir,install,mkfifo,mknod: reject octal clauses and empty modes in mode lists

[krosci]

Fixes #14421.

The mode parser accepted an octal clause anywhere inside a comma-separated list and skipped empty clauses, where GNU only allows a bare octal as the entire mode string and rejects empty clauses. The comma-list loop was duplicated in chmod, so the grammar now lives once in uucore::mode, with chmod routed through it via parse_chmod_with_naive to keep its umask diagnostic; a caret-aware clause_start in ModeError lets a diagnostic point at the rejected clause. Tests cover both tables from the issue for chmod, mkdir, install, mkfifo and mknod.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/chmod/setgid. tests/chmod/setgid is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/date/resolution (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/inotify-dir-recreate (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/misc/tty-eof (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)
Congrats! The gnu test tests/unexpand/bounded-memory is now passing!