audio/filter: rename af_force.c to af_format.c

The filter itself was already renamed earlier - but rename the file too.
af_force: minor simplifications
af_force: set format early for better debug output

Set the input/output format in filter init. This doesn't change anything
functionally, but it makes the forced format show up in the filter chain
init verbose output (which sometimes prints the filter chain before all
filters have been configured).
audio/filter: remove useless af_info fields

Drop the author and comment fields. They were completely unused - not
even printed in verbose mode, just dead weight.

Also use designated initializers and drop redundant flags.
video/out: remove useless info struct and redundant fields

The author and comment fields were printed only in -v mode.
video/filter: remove useless vf_info fields

This time I didn't bother to move the contents of the author field to
the file headers. "git log" is your friend.
audio/out: remove useless info struct and redundant fields
manpage: use the new name for af_force in one place