ext/alsa/: Use alsa trigger_tstamp to get the timestamp of the first sample in the buffer for more precise sync. Some...

Original commit message from CVS:
* ext/alsa/gstalsa.c: (gst_alsa_start), (gst_alsa_xrun_recovery):
* ext/alsa/gstalsa.h:
* ext/alsa/gstalsasrc.c: (gst_alsa_src_init),
(gst_alsa_src_update_avail), (gst_alsa_src_loop):
Use alsa trigger_tstamp to get the timestamp of the first
sample in the buffer for more precise sync. Some cleanups.