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.

Here's a sample:

    $ ostree admin --help
    Usage:
      ostree admin [OPTION...] --print-current-dir|COMMAND

    Builtin "admin" Commands:
      cleanup
      config-diff
      deploy
      init-fs
      instutil
      os-init
      status
      switch
      undeploy
      upgrade

    Help Options:
      -h, --help         Show help options

    Application Options:
      --sysroot=PATH     Create a new OSTree sysroot at PATH
      -v, --verbose      Print debug information during command processing
      --version          Print version information and exit

https://bugzilla.gnome.org/show_bug.cgi?id=740295