start to add some SOUND_TRIGGER_ACHIEVEMENTS code

Start to add supporting code to windsound and macsound. The latter
remains commented out because I haven't had a chance to try
it on macOS yet.

In order to test it out, I added two more stock sounds:
sa2_xplevelup and sa2_xpleveldown.

include/sndprocs.h

@@ -335,8 +335,8 @@ enum ambiences {
 };
 
 enum achievements_arg2 {
-    sa2_splashscreen, sa2_newgame_nosplash, sa2_restoregame,
-    sa2_xplevelup, sa2_xpleveldown
+    sa2_zero_invalid, sa2_splashscreen, sa2_newgame_nosplash, sa2_restoregame,
+    sa2_xplevelup, sa2_xpleveldown, number_of_sa2_entries
 };
 
 /*