alsa: add api.alsa.headroom setting

Can be used to tweak the extra distance kept between the hardware
and software pointers when reading and writing.

See #289

src/daemon/media-session.d/alsa-monitor.conf

@@ -50,9 +50,12 @@ rules = [
                 #resample.quality = 		4
                 #channelmix.normalize =		false
                 #channelmix.mix-lfe = 		false
+                #audio.channels = 		2
                 #audio.format = 		"S16LE"
                 #audio.rate = 			44100
                 #audio.position = 		"FL,FR"
+                #api.alsa.period-size =         1024
+                #api.alsa.headroom =            0
             }
         }
     }