gst/: Better debugging of clocking info.

gst/: Better debugging of clocking info.

Original commit message from CVS:
* gst/base/gstbasesink.c: (gst_basesink_base_init),
(gst_basesink_pad_getcaps), (gst_basesink_init),
(gst_basesink_do_sync):
* gst/gstclock.c: (gst_clock_entry_new):
* gst/gstevent.c: (gst_event_discont_get_value):
* gst/gstpipeline.c: (pipeline_bus_handler),
(gst_pipeline_change_state):
* gst/gstsystemclock.c: (gst_system_clock_id_wait_unlocked):
Better debugging of clocking info.
Allow NULL values when getting discont values.