filesink: Add a new full buffer mode to filesink

filesink: Add a new full buffer mode to filesink

Previously the default and full modes were the same. Now the default
mode is like before: it accumulates all buffers in a buffer list until
the threshold is reached and then writes them all out, potentially in
multiple writes.

The new full mode works by always copying memory to a single memory area
and writing everything out with a single write once the threshold is
reached.