v4l2: add format negotiation and OpenBSD support

Changes are:
 * libv4l2 is not required for proper operation any more
   (it is still desirable, as it guarantees that librem-supported format will
   be available);
 * v4l2 module now iterates through the list of available formats and settles
   on first one supported by librem;
 * if libv4l2 is present, baresip will try to use native format and fall back
   to emulated one only if no usable combination found.
whitespace
v4l2: remove trailing whitespace