This updates the copyright header to all be the same, which is
pretty much an update of where to mail request for a copy of the GPL
and the years of the MPD project. This also puts all committers under
'The Music Player Project' umbrella. These entries should go
individually in the AUTHORS file, for consistancy.

"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.

This will make it easier to introduce locking

dirvec_find() does not modify the object, thus it should get a const
pointer.

No idea why it was created in directory.h, but it should be in
dirvec.h.

The struct can be forward-declared by other headers, which relaxes the
header dependencies.

Having all functions as static (non-inline) functions generates GCC
warnings, and duplicates binary code across several object files.
Most of dirvec's methods are too complex for becoming inline
functions.  Move them all to dirvec.c and publish the prototypes in
dirvec.h.

Small memory reduction compared to songvec since most users have
much fewer dirs than songs, but still nice to have.