rygel: Introduce -r/--replace option

Also change default behavior: Previously, rygel would silently replace an
already running instance. Now it just exists. To return to old behavior, the
-r/--replace option was introduced.

Signed-off-by: Jens Georg <mail@jensge.org>

https://bugzilla.gnome.org/show_bug.cgi?id=731859