Let GLib produce usage information

The command line option parser in GLib can automatically create usage
information that is presented when the --help flag is passed to the
program. Enable this flag in the command line parser.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Set descriptions of our command line options

We should set descriptions of the command line options, otherwise the
command line option help isn't very helpful except for listing possible
options. Add descriptions to all options.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>