mp4: Replace mp4ff prefix by mp4.

mp4: Replace mp4ff prefix by mp4.

The additional "ff" does not convey any useful information.

This patch was produced by running

	sed -i 's/mp4ff/mp4/g' mp4.c mp4.h aac_afh.c

followed by

	sed -i 's/MP4FF/MP4/g' aac_afh.c error.h

and the manual removal of the "ff" suffix in the error strings.