zpool/zfs: accept --help, -h, -? after a subcommand

[chrislongros]

Motivation and Context

zpool <subcommand> --help (and -h, -?) prints invalid option '-' followed by the subcommand's usage line, instead of showing help. Same bug for every subcommand. First reported as issue 1 in #17603.

Description

Catch --help, -h, and -? after a subcommand, look it up in the command table, and print its short usage. Unknown subcommands fall through to the existing "unrecognized command" error.

How Has This Been Tested?

zpool scrub|checkpoint|create|status|attach|list --help|-h|-? on:

• Arch Linux 6.19.10 — prints the matching short usage and exits 0.
• FreeBSD 16.0-CURRENT — same.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[behlendorf]

It looks like we a few consistent failures which will need to be looked at.

Tests with results other than PASS that are unexpected:
    FAIL cli_root/zfs_diff/zfs_diff_cliargs (expected PASS)
    FAIL cli_root/zfs_diff/zfs_diff_mangle (expected PASS)
    FAIL rsend/send_holds (expected PASS)