Remember the modification time of each directory. This is important
for archives (which are virtual directories right now), but may also
be useful for an automatic update mechanism.
Don't use db_get_directory() and traverse the full path with every
directory being loaded. Just see if the current parent contains the
entry. Everything else would be invalid anyway..
A manipulated database could trigger an assertion failure, because the
parent didn't match. Do a proper check if the new directory is within
the parent's. This uses FATAL() to bail out, so MPD still dies, but
it doesn't crash.
Remove clutter from directory.c. Everything which saves or loads
to/from the hard disk goes to directory_save.c, and code which sends
directory information to the client is moved into directory_print.c.
