Support POSIX-style tar keystring syntax (no leading dash)

[Copilot]

POSIX specifies tar key [file...] where the first operand is a keystring with no - prefix (e.g. tar cvf archive.tar files), but the implementation only accepted GNU-style dash-prefixed flags.

Approach

Rather than rewriting the clap argument definitions, the args vector is pre-processed before clap sees it: if args[1] is a valid POSIX keystring it is expanded in-place into equivalent dash-prefixed flags.

tar cvf archive.tar file.txt   →   tar -c -v -f archive.tar file.txt
tar xvf archive.tar            →   tar -x -v -f archive.tar

The f and b modifiers consume file operands in key-character order, matching the POSIX ordering requirement. Both styles remain fully supported.

Changes

• is_posix_keystring(s) — detects a valid keystring: no leading -, at least one function letter (c x t u r), all chars from the recognised key-character set.
• expand_posix_keystring(args) — rewrites Vec<OsString> when args[1] is a keystring; f and b consume positional file operands in order; remaining operands pass through unchanged.
• uumain — calls expand_posix_keystring after the existing util-name-skipping step.
• USAGE string — updated to show both invocation forms.

Tests

• 13 unit tests for is_posix_keystring and expand_posix_keystring covering acceptance, rejection, and expansion edge cases.
• 5 integration tests: cf, cvf, xf, xvf end-to-end, plus a round-trip assertion that POSIX and dash-prefix styles produce bit-identical archives.

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