avformat/hashenc: use an array of hashes

avformat/hashenc: use an array of hashes

Only the first element of the array is used currently, the other
elements are in preparation for a new muxer calculating multiple
hashes.

Also move alloc/init code from the write_header() functions to
dedicated init() functions, and the cleanup code from the
write_trailer() functions to dedicated deinit() functions.

hash_free() and framehash_free() turn out to be identical here,
but will differ in the subsequent commit, so they are not consolidated.

Signed-off-by: Moritz Barsnick <barsnick@gmx.net>
Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>