Show capability list with --version option
Simple help output corrections.
Refactor command-line parsing.

Refactor command-line parsing to better utilize GOptionContext.  This
eliminates most of the manual parsing and global options are now shown
in the help output.

Some of the changes here are not strictly necessary for rpm-ostree,
but are done for consistency with ostree's command-line parsing.

The "rpm" subcommand needs some extra attention, so that's been split
into a separate commit.
Refactor command-line parsing for "rpm" command.

Similar to the previous commit, but the "rpm" command needed some extra
attention.

I stopped short of splitting the subcommands into separate files (like
for "compose"), but refactored the parsing as though they were separate.
Remove some unnecessary function parameters

Starting with pkg_yumdb_strdup(), the GError parameter is unused and
therefore (arguably) the GCancellable parameter is also not needed.
Remove them both, and clean up other functions that now have unused
parameters as a result.

Note that none of the callers of these functions were checking for
errors anyway.
RpmOstreeCommand: Remove unused 'flags' member

There are currently no command flags defined, but if we need any they
would likely be passed to the custom GOptionContext parse function as
in OSTree.