libs/gst/base/: Add ::unlock_stop to basesrc and basesink. This allows an opportunity for sub-classes to correctly cl...

Original commit message from CVS:
* libs/gst/base/gstbasesink.c: (gst_base_sink_set_flushing),
(gst_base_sink_change_state):
* libs/gst/base/gstbasesink.h:
* libs/gst/base/gstbasesrc.c: (gst_base_src_perform_seek),
(gst_base_src_default_event), (gst_base_src_unlock_stop),
(gst_base_src_deactivate):
* libs/gst/base/gstbasesrc.h:
Add ::unlock_stop to basesrc and basesink. This allows an opportunity
for sub-classes to correctly clear any state they set trying to
unlock, such as clearing out unlock commands from a command fd.
* plugins/elements/gstfdsink.c: (gst_fd_sink_class_init),
(gst_fd_sink_render), (gst_fd_sink_unlock),
(gst_fd_sink_unlock_stop):
* plugins/elements/gstfdsrc.c: (gst_fd_src_class_init),
(gst_fd_src_init), (gst_fd_src_unlock), (gst_fd_src_unlock_stop),
(gst_fd_src_create), (gst_fd_src_get_size), (gst_fd_src_do_seek):
Implement unlock_stop in fdsrc and fdsink.
Implement seeking in fdsrc when a seekable fd is passed, as in
gst-launch-0.10 fdsrc ! ... ! xvimagesink < /path/to/file