dos port chdir fix

I don't know if this was introduced post 3.4.3 or not, but the
DOS port was doing a chdir at the start, but not doing one
at the finish of the game, so you ended up in the same
directory as the NetHack.exe executable post-game.

It was doing the chdir even if NOCWD_ASSUMPTIONS was
defined.

sys/share/pcmain.c

@@ -167,7 +167,7 @@ char *argv[];
 			fqn_prefix[prefcnt] = fqn_prefix[0];
 		}
 #endif
-#ifdef CHDIR
+#if defined(CHDIR) && !defined(NOCWD_ASSUMPTIONS)
 		chdirx (dir, 1);
 #endif
 	}