gdbus-tool: Factor out common GOptionContext construction
gdbus-tool: Factor out common GOptionContext construction

In doing so, ensure that g_option_context_set_ignore_unknown_options()
is always called if completion is being done.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=793597

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib