machine-id-commit: merge machine-id-commit functionality into machine-id-setup

machine-id-commit: merge machine-id-commit functionality into machine-id-setup

And remove machine-id-commit as separate binary.

There's really no point in keeping this separate, as the sources are
pretty much identical, and have pretty identical interfaces. Let's unify
this in one binary.

Given that machine-id-commit was a private binary of systemd (shipped in
/usr/lib/) removing the tool is not an API break.

While we are at it, improve the documentation of the command substantially.