Consume mode removes each song played

In the MPD_AUTO_DISABLED() function, I forgot to check for "explicitly
disabled".

Ths usual defaults updates, this includes clarifications, cleanups, updates
and fixes.

MPD_AUTO_PKG() runs pkg-config and then forwards control to
MPD_AUTO_RESULT().  This is a commonly used short cut.

Don't add those to MPD_CFLAGS and MPD_LIBS.

Don't add those to MPD_CFLAGS and MPD_LIBS.

Don't add those to MPD_CFLAGS and MPD_LIBS.

This patch adds a small autoconf M4 library which deals with
auto-detected features.  The default for those features is "auto",
which is like the old default: if the library is present on the
system, enable the feature, disable otherwise.  If the user explicitly
enables that feature (--enable-alsa), and the library is not present,
configure must fail, because it cannot fulfill the request.

Previous cast to float didn't have any effect because one value is uint
and the other is a floating type but the number itself is even..
This caused some tracks to end before they were really at an end.

Added an assertion in mixer_set_volume().  Removed the range checks
from the ALSA and OSS plugins.

The "mpcdec" plugin is based on the libmpcdec library.

Renamed variables and functions.

The strings passed to tag_pool_get_item() are not null-terminated, and
the caller passes the string length.  Don't assume it is
null-terminated anyway by using strcmp().

We're using GLib macros which were introduced in GLib 2.6, like
G_GNUC_MALLOC.  Let's just drop support for GLib versions older than
2.6 instead of adding more complicated compatibility hacks.

socket, because there is no ioctl for non-sockets on Windows

networking, select(), ntohl(), etc.

Simplify initialization and finalization.

There are numerous race conditions between the libpulse thread
(pulse_mixer.c callbacks) and the rest of MPD.  Protect the volatile
attributes of the pulse_mixer struct with a mutex to fix that.

Eliminate one indent level.  Also remove several debug useless debug
messages.

Those parameters are used after all.

Don't mess with pulse_mixer.volume for setting the volume.  This
variable should only be used to transfer the current volume from
sink_input_vol() to pulse_mixer_get_volume().

The pa_context_get_sink_input_info() function is asynchronous, and
after it returns, libpulse does not guarantee that the operation has
completed yet; in fact, it is not likely.  Explicitly wait for the
operation to complete.

The code for the new pulse_wait_for_operation() function was inspired
by mplayer and xine code.

The function mixer_failed() expects the mixer mutex to be already
locked, yet it calls mixer_close(), which attempts to lock the mutex
again, deadlocking itself.

When single mode is enabled, after current song it stops playback,
or it replay same song if repeat mode is activated.

The smartstop feature is a way to tell mpd to stop playing after
current song.
This patche provides:
- 'state' command returns 'smartstop' state (1 or 0)
- 'smartstop' can activate or not the smartstop state
- when song is terminated, mpd stops playing and smartstop is set to 0