g_application_add_main_option: fix type signature

The flags argument is a GOptionFlags so use that type instead of 'int'.
goption: Add G_OPTION_FLAG_NONE

This is helpful to better document code, as G_OPTION_FLAG_NONE is more
readable than 0.
GApplication:handle-local-options: document return value

The return value for this signal was documented in the prose, but not
properly in a Returns: stanza.  Fix that.