Instead of printing out the Id from playlist.c, instead set
the integer that added_id poitns to if added_id is non-NULL.

This makes the API cleaner and will allow us to use additional
commands to manipulate the newly-added song_id.  Callers
(handleAddId) that relied on printId to print it to the given
fd have now been modified to print the ID at a higher-level;
making playlist.c less-dependent on protocol details.

git-svn-id: https://svn.musicpd.org/mpd/trunk@7149 09075e82-0dd4-0310-85a5-a0d7c8717e4f