Remove GLib dependency from some headers.

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

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.

On some platforms, g_free() must be used for memory allocated by
GLib.  This patch intends to correct a lot of occurrences, but is
probably not complete.

Only include headers which are really needed.  os_compat.h aimed to
make MPD easily portable, but was never actually made portable.

We want to remove gcc.h eventually. This takes care of all the
G_GNUC_MALLOC macros.

Due to a minor typo, the string set had duplicate values, because
strset_add() didn't check the base slot properly.

"struct strset" is a hashed string set: you can add strings to this
library, and it stores them as a set of unique strings.  You can get
the size of the set, and you can enumerate through all values.

This will be used to replace the linear tagTracker library.