This modifies the string in place, and does not allocate any memory from
the heap.  This is considerably smaller than the function it replaces,
and will be instrumental in getting the commands/conf malloc reductions
done.

git-svn-id: https://svn.musicpd.org/mpd/trunk@4481 09075e82-0dd4-0310-85a5-a0d7c8717e4f