v4l2allocator: use v4l2_requestbuffers capabilities to determine allocator flags

There is a new capabilities field in v4l2_requestbuffers that can tell us whether the V4L2 queue supports MMAP, USERPTR, and DMABUF buffers. If any of those flags is set in the first REQBUFS(0) result, we can use those to set allocator flags and stop probing in gst_v4l2_allocator_new.

A kernel header update with the new flags is contained in !136 (merged).