"LOG_H" is a macro which is also used by ffmpeg/log.h.  This is
ffmpeg's fault, because short macros should be reserved for
applications, but since it's always a good idea to choose prefixed
macro names, even for applications, we are going to do that in MPD.

When there are standardized headers, use these instead of the bloated
os_compat.h.

Again, a data type which can be forward-declared.

Pass const songvec pointers to songvec_find() and songvec_for_each().

This is so we can more consistently deal with locking
needed for thread-safety in iterator functions.

We don't modify the Song when we delete it

"free" implies the songvec structure itself is freed,
which is not the case.

Any pruned files will be noticed during update and pruned
from the live database, so this inefficient function can
go away and never come back.

Our linked-list implementation is wasteful and the
SongList isn't modified enough to benefit from being a linked
list.  So use a more compact array of song pointers which
saves ~200K on a library with ~9K songs (on x86-32).