mkosi: rework --help to print command syntax
mkosi: rework --help to print command syntax

Command list and syntax is the most interesting part of the output.
But because of how argparse works, we were showing this part very
briefly and buried in other options. Instead of relying on the autogenerated
description, let's reuse the list from the man page.
Because a literal text is added, the lines for verbs and --help
and --version are suppressed.

I changed "Distribution" to "Distribution options" and so on for each
option group. The additional word connects the synopsis (which says
"[options…]" with the list below.

Output:
$ bin/mkosi --help| head -n 20
usage:
       mkosi [options...] summary
       mkosi [options...] build [script parameters...]
       mkosi [options...] shell [command line...]
       mkosi [options...] boot [nspawn settings...]
       mkosi [options...] qemu [qemu parameters...]
       mkosi [options...] ssh [command line...]
       mkosi [options...] clean
       mkosi [options...] serve
       mkosi [options...] bump
       mkosi [options...] genkey
       mkosi [options...] help
       mkosi -h | --help
       mkosi --version

Build Bespoke OS Images

Distribution options:
  -d, --distribution {fedora,debian,...

When printed on the console, summary/build/shell/… are bold.

systemd/mkosi,systemd/mkosi