This also does the following changes:
(1) pull the bufferpool code out into gstv4l2bufferpool.c, and make a
    bit more generic so it can be used both for v4l2src and v4l2sink
(2) move some of the device probing/configuration/caps stuff into
    gstv4l2object.c so it does not have to be duplicated between
    v4l2src and v4l2sink

Fixes bug #590280.