[main] hadd: properly handle help flag and add tests

[silverweed]

This Pull request:

makes hadd handle -h, -help or --help like the other flags, i.e. accepting it in any valid position rather than just as the first flag.
Note that we accept all versions because a user that requires help on the command shouldn't have to guess the proper form of the flag, we just accept all canonical ones.

Checklist:

• tested changes locally
• updated the docs (if necessary)

[github-actions]

Test Results

    21 files      21 suites   3d 15h 7m 55s ⏱️
 3 698 tests  3 698 ✅ 0 💤 0 ❌
75 794 runs  75 794 ✅ 0 💤 0 ❌

Results for commit 5d2c615.

♻️ This comment has been updated with latest results.

[pcanal]

Thanks.