games/doom: fix switch textures swapping in ultimate doom (thansk qu7uux)

gamemode is set according to the name of the main wad (cf. d_main.c), i.e.:
- doom1.wad: (shareware doom1, ep1 only) gamemode == shareware
- doom.wad: (registered doom1, ep1-3) gamemode == registered
- doomu.wad: (ultimate doom, ep1-4) gamemode == retail
- doom2.wad, plutonia.wad, tnt.wad: gamemode == commercial
most doom.wad's distributed online are, in fact, ultimate doom.
if your ultimate doom wad is correctly named doomu.wad, some switches in
episodes 2-4 won't swap their texture when toggled, because
p_switch.c:P_InitSwitchList() is only checking for registered doom1.

easy way to test: demo2 in either registered or ultimate doom: the player flips
a switch right at the beginning of the demo; if the main wad is called
doomu.wad, the switch won't change its texture.
% games/doom -playdemo demo2
if you rename the wad to doom.wad or alter d_main.c:IdentifyVersion, the switch
will swap its texture like it should.

sys/src/games/doom/p_switch.c

@@ -112,7 +112,7 @@ void P_InitSwitchList(void)
 	
     episode = 1;
 
-    if (gamemode == registered)
+    if (gamemode == registered || gamemode == retail)
 	episode = 2;
     else
 	if ( gamemode == commercial )