gst/gstelement.c: Fix to allow DELAYED to indicate that linking was successful.

gst/gstelement.c: Fix to allow DELAYED to indicate that linking was successful.

Original commit message from CVS:
* gst/gstelement.c: (gst_element_link_pads_filtered),
(gst_element_negotiate_pads): Fix to allow DELAYED to indicate
that linking was successful.
* gst/gstpad.c: (gst_pad_link_free),
(gst_pad_link_call_link_functions), (gst_pad_link_negotiate),
(gst_pad_link_try), (gst_pad_link_unnegotiate),
(gst_pad_unnegotiate), (gst_pad_set_explicit_caps): Pass
GstPadLinkReturn correctly between functions, and don't fail
when DELAYED is used (DELAYED is very important).  Better
cleanup on unlinking and unnegotiation.  Should fix some spider
bugs.