sys/v4l/: Add new v4ljpegsrc for handling the ov51x hacky "I'll give you jpeg inside rgb frames" driver.

Original commit message from CVS:
* sys/v4l/gstv4l.c: (plugin_init):
* sys/v4l/gstv4ljpegsrc.c: (gst_v4ljpegsrc_get_type),
(gst_v4ljpegsrc_base_init), (gst_v4ljpegsrc_class_init),
(gst_v4ljpegsrc_init), (gst_v4ljpegsrc_src_link),
(gst_v4ljpegsrc_getcaps), (gst_v4ljpegsrc_get):
* sys/v4l/gstv4ljpegsrc.h:
* sys/v4l/gstv4lsrc.c: (gst_v4lsrc_open), (gst_v4lsrc_src_link):
* sys/v4l/v4l_calls.h:
* sys/v4l/v4lsrc_calls.c: (gst_v4lsrc_palette_name),
(gst_v4lsrc_get_fps):
* sys/v4l/v4lsrc_calls.h:
Add new v4ljpegsrc for handling the ov51x hacky "I'll give
you jpeg inside rgb frames" driver.
Don't error in the v4lsrc link function, just return
REFUSED.