Don't use the global variables from the DatabaseSimple library.

Useless information.

Move global variables into the new classes.  That may allow multiple
update threads for multiple databases one day.

Allows the kernel to combine timer wakeups with other processes,
reducing power usage.

Don't use global variables.

Another big chunk of code for multi-player support.

This commit adds the NeighborPlugin API which can be used to detect
nearby file servers that can be used by input plugins.  This list of
servers is exported using the new "listneighbors" command.  The idle
even "neighbor" notifies interested clients when a new neighbor is
found or an existing one is lost.

There's a lot missing currently: protocol&user documentation, and a
way to "mount" remote servers into the music database.  Obviously,
some code from the UPnP database plugin can be moved to a neighbor
plugin.

Not a fatal error.  Fixes assertion failure.

Keep the parent process around until MPD has finished initializing.

This is important for libraries that are allergic to fork(), such as
libupnp.

Allow database plugins to announce that they have been modified.

Allows building without GLib.  This fails to compile currently,
because GLib is still used in the MPD core.

Wrapper for pcm_resample_global_init(), just in case other PCM
libraries need initialization, too.

Otherwise sockets can't be created during IO thread initialization.

Reduce GLib usage.

Map LogLevel::INFO to G_LOG_LEVEL_INFO, and LogLevel::DEFAULT to
G_LOG_LEVEL_MESSAGE.  Now client connect/disconnect message are only
logged on log_level "secure".

2 MB was too small for cross-fading a 24 bit file.  Increasing to 4 MB
is still not too large for weak machines, but is enough for
cross-fading.

Fixes crash.