mp4: Implement error checking for the write path.

mp4: Implement error checking for the write path.

Although the ->write callback has a return value, it is of unsigned
type and is never checked. Fix this by changing the prototype to
match that of the write(2) system call, check the return value of the
callback in the write_data() wrapper of mp4.c and propagate paraslash
error codes back to aac_afh.c via the public mp4_meta_update().

While at it, handle short writes and EINTR properly, and fix the
indentation of the callback structure in mp4.h.