[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

    22 files      22 suites   3d 20h 12m 3s ⏱️
 3 697 tests  3 696 ✅ 0 💤 1 ❌
79 383 runs  79 378 ✅ 0 💤 5 ❌

For more details on these failures, see this check.

Results for commit d6441ea.