Add UpdateQueueItem::id to keep track of the id in every item.
Replaces thhe hack in update_queue_push().

Fixes a few kludges and avoids GLib allocation.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.

More preparations for multi-player support.

Fixes deprecation warnings.

This is the beginning of multi-player support.  There will be support
for multiple Partition objects in one MPD process.

Preparing for non-"simple" database plugins.

Replaced all occurrences of g_error() with MPD_ERROR() located in a new header
file 'mpd_error.h'. This macro uses g_critical() to print the error message
and then exits gracefully in contrast to g_error() which would internally call
abort() to produce a core dump.

The macro name is distinctive and allows to find all places with dubious error
handling. The long-term goal is to get rid of MPD_ERROR() altogether. To
facilitate the eventual removal of this macro it was added in a new header
file rather than to an existing header file.

This fixes #2995 and #3007.

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.

All sources which might work with large files must include config.h,
to get Large File Support on 32 bit platforms.

That variable has been superseded by "remove_notify" (defined in
update_remove.c).

When you disable the "follow_outside_symlinks" or the
"follow_inside_symlinks" setting, the next update should remove the
now-ignored files from the database.

The return value of map_directory_child_fs() must be freed.

Removed the NULL check.  If that NULL check was correct, that would
have been a memory leak (vtrack).

"rescan" is the same as "update", but it discards existing songs in
the database.