sys/v4l/: Added a sync mode enum property to control v4lsrc timestamp method

sys/v4l/: Added a sync mode enum property to control v4lsrc timestamp method

Original commit message from CVS:
* sys/v4l/gstv4lsrc.c: (gst_v4lsrc_sync_mode_get_type),
(gst_v4lsrc_class_init), (gst_v4lsrc_init), (gst_v4lsrc_get_fps),
(gst_v4lsrc_get), (gst_v4lsrc_set_property),
(gst_v4lsrc_get_property):
* sys/v4l/gstv4lsrc.h:
* sys/v4l/v4l-overlay_calls.c: (gst_v4l_set_overlay):
Added a sync mode enum property to control v4lsrc timestamp method
Removed the use-fixed-fps property and moved functionality in
the enum.
Don't error on an error value from v4l-conf, it might not always
be a real error.