Add libv4l2 as requirement for the v4l2 plugin.

In order to convert formats not natively supported by obs the
v4l2 userspace library is required. This patch adds a cmake
script to find the library.
Replace all system calls with there counterpart in libv4l2.

By using the wrapper functions supplied by libv4l2 we gain
support for formats not natively supported by obs.
The library intercepts certain system calls to transparently
enable recoding.
Display emulated formats as such in v4l2 plugin.
Fix handling of multiplanar pixelformats in v4l2 plugin.

The data provided by v4l2 for multiplanar formats is one
consecutive block that just needs to be splitted for obs.