GApplication: add --gapplication-service switch

Add a --gapplication-service switch to the default implementation of
local_command_line.  This name is unlikely to clash with any option used
by an existing application.

When a normal application (neither service nor launcher) is launched with
exactly this one argument, G_APPLICATION_IS_SERVICE will be set.

The idea is that people will write their D-Bus service file with
--gapplication-service on the Exec line.  This provides a nice
compromise for people who want the benefits of DBusActivatable
applications but without losing the ability to easily run them directly
(under the debugger or inside jhbuild, etc.)

https://bugzilla.gnome.org/show_bug.cgi?id=710965