Add "option parser" infrastracture that helps with cmdline option parsing

Add "option parser" infrastracture that helps with cmdline option parsing

The basic idea is that we'll have "one source of truth" for the list of
options. Currently, this is split between:
  1. struct option options[] array for long options
  2. the short option parameter to getopt_long()
  3. --help
so it is easy to forget to add or update one of those places where
appropriate.

An option is defined through a macro that includes the option short
and long codes, and also the metavar and help. Those four items can
be used to generate the help string automatically.

The code is easier to read when various parts are written in the same
order.

We can define common options through a macro in the header file,
reducing boilerplate repeated in different files. Over time, if we
discover that the same pattern is used in multiple files, we can add
another "common option".

The macro is defined in a way that the editor can indent it like a
normal case statement.

The error message for ambiguous options is formatted a bit differently:
$ systemd-id128 --no-
systemd-id128: option '--no-' is ambiguous; possibilities: '--no-pager' '--no-legend'
$ build/systemd-id128 --no-
option '--no-' is ambiguous; possibilities: --no-pager, --no-legend

I think the formatting without commas is ugly, but OTOH, the quotes
around option names are superfluous, real option names are easy to
distinguish.