gst/gstevent.c: Rename some more @cur to @start to fix docs.

gst/gstevent.c: Rename some more @cur to @start to fix docs.

Original commit message from CVS:
* gst/gstevent.c: (gst_event_new_seek), (gst_event_parse_seek):
Rename some more @cur to @start to fix docs.
* gst/gstsegment.c: (gst_segment_set_seek):
Fix typo.
time and start must always stay in sync as defined in design doc.
* gst/gsttaglist.c: (gst_tag_list_is_empty):
Rename param to fix docs.
* tests/check/gst/gstsegment.c: (GST_START_TEST):
Check that start and time are in sync.
* tests/check/pipelines/parse-launch.c:
(gst_parse_test_element_change_state):
Activate pad before adding to the element.