Move from PcmClamp().

I've created an elementary input plugin that plays sound from the
soundcard, so you can use MPD to listen to anything connected to the
line-in jack, or to Video4Linux FM radio cards that send audio through
the soundcard.  There has been a small number of posts here in the
past requesting line-in input, so here is a first, simplistic stab at
it.

The patch adds a new sheme, alsa://, which causes mpd to play data
read directly from a souncdard.  It defaults to hw:0,0, but you can
pass any ALSA device name in the URI.  So, using mpc for example:

 mpc add alsa://
 mpc play

will play from device hw:0,0.

To use a diffferent device:

 mpc add alsa://hw:1,0

Due to recent protocol additions ("addtagid", "cleartagid").

The DynamicFifoBuffer methods never return nullptr when the buffer is
empty or full; instead, they return an empty buffer.  This bug caused
an endless loop.

Switch to the C++ version.

This deadlock was a regression by commit 8e38b4f8.  Since we currently
can't resolve this, let's revert the commit, and add a GLib specific
workaround for the build failure.

Avoid GLib.

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

Avoid GLib.

Prepare for GLib removal.

Replaces g_uri_parse_scheme().