Fix bug when starting multiplayer Chex Quest games.

The server was rejecting connects when running with chex.wad, because it
was expecting the gamemode to be shareware; in fact, chex.wad contains
four episodes worth of levels (even though only five levels are used).
Technically it is therefore a retail IWAD and we should expect
gamemode=retail when using it. This fixes #647; thanks to Danfun64 for
reporting the bug.

src/d_mode.c

@@ -30,7 +30,7 @@ static struct
     int episode;
     int map;
 } valid_modes[] = {
-    { pack_chex, shareware,  1, 5 },
+    { pack_chex, retail,     1, 5 },
     { doom,      shareware,  1, 9 },
     { doom,      registered, 3, 9 },
     { doom,      retail,     4, 9 },