pcm_volume: added constant PCM_VOLUME_1

It may be desirable to change the range of integer volume levels
(e.g. to 1024, which may utilize shifts instead of expensive integer
divisions).  Introduce the constant PCM_VOLUME_1 which describes the
integer value for "100% volume".  This is currently 1000.

src/volume.c

@@ -480,7 +480,7 @@ static int changeSoftwareVolume(int change, int rel)
 
 	/*new = 100.0*(exp(new/50.0)-1)/(M_E*M_E-1)+0.5; */
 	if (new >= 100)
-		new = 1000;
+		new = PCM_VOLUME_1;
 	else if (new <= 0)
 		new = 0;
 	else