server: Convert com_ls() to lopsub.

server: Convert com_ls() to lopsub.

This is the final server command to be converted to the lopsub API.
Besides the conversion to lopsub, the patch changes the semantics
of the ls command in two ways: The deprecated -p option is removed,
and the -F option is made the default so that full paths are printed
if -b is not given.

Regarding the conversion, the patch removes enum ls_flags which used to
contain the various flags of the ls command. This information can now
be extracted in the callback from the deserialized parse result. Also
write_score() is kind of pointless and is replaced by inlining it.

Note that make_status_items() is also affected by this patch because
it makes use of the infrastructure of the ls command. This function
is called from the event handler of the audio file table for events
of type AFSI_CHANGE and AFHI_CHANGE.

Since no more non-lopsub server commands are left after this patch,
several cleanups are possible. These are dealt with in subsequent
patches.