detect v4l2 input on freebsd

On FreeBSD there's multimedia/webcamd port that provides a bunch of linux kernel drivers in userland. Many of them are for webcams and some don't support pixel formats that mpv understands e.g., V4L2_PIX_FMT_JPEG-only cams.

webcamd also supports a few PVR and Radio drivers.

http://www.freshports.org/multimedia/webcamd
http://web.archiveorange.com/archive/v/JZyvuczivPuypyze5Rhd (history)

It's an attempt to upstream this patch:

http://svnweb.freebsd.org/ports/head/multimedia/mpv/files/patch-stream-tvi_v4l2.c
http://svnweb.freebsd.org/ports/head/multimedia/mplayer2/files/patch-stream-tvi_v4l2.c
http://svnweb.freebsd.org/ports/head/multimedia/mplayer/files/patch-stream-tvi_v4l2.c