add draft ALSA input plugin

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

src/ls.cxx

@@ -54,6 +54,9 @@ static const char *remoteUrlPrefixes[] = {
 #endif
 #ifdef ENABLE_DESPOTIFY
 	"spt://",
+#endif
+#ifdef HAVE_ALSA
+	"alsa://",
 #endif
 	NULL
 };