plugins/elements/gstinputselector.*: Figure out the locking a bit more.

plugins/elements/gstinputselector.*: Figure out the locking a bit more.

Original commit message from CVS:
* plugins/elements/gstinputselector.c:
(gst_selector_pad_get_running_time), (gst_selector_pad_reset),
(gst_selector_pad_event), (gst_selector_pad_bufferalloc),
(gst_input_selector_wait), (gst_selector_pad_chain),
(gst_input_selector_class_init), (gst_input_selector_init),
(gst_input_selector_dispose), (gst_segment_set_start),
(gst_input_selector_set_active_pad),
(gst_input_selector_set_property),
(gst_input_selector_get_property),
(gst_input_selector_get_linked_pad),
(gst_input_selector_is_active_sinkpad),
(gst_input_selector_activate_sinkpad),
(gst_input_selector_request_new_pad),
(gst_input_selector_release_pad),
(gst_input_selector_change_state), (gst_input_selector_block),
(gst_input_selector_switch):
* plugins/elements/gstinputselector.h:
Figure out the locking a bit more.
Mark buffers with discont after switching.
Fix initial segment forwarding, make sure to only forward one segment
regardless of what the sequence of buffers/segments is. See #522203.
Improve flushing when blocked.
Return NOT_LINKED when a stream is not selected.
Not API change for the switch signal in the docs.
Fix start/time/accum values of the new segment.
Correctly unlock and flush a blocking selector when going to READY.