gst/multipart/multipartmux.c: Added configurable boundary specifier, added the value as a caps field as well.

Original commit message from CVS:
* gst/multipart/multipartmux.c: (gst_multipart_mux_class_init),
(gst_multipart_mux_init), (gst_multipart_mux_loop),
(gst_multipart_mux_get_property), (gst_multipart_mux_set_property),
(gst_multipart_mux_change_state):
Added configurable boundary specifier, added the value as a
caps field as well.