improve selectsave handling for Windows

If there were outdated savefiles encountered during
startup, each individual one was getting a wait_synch
that required a <return> even though a message window
wasn't being used at that point.

Allow suppression of the individual per-file wait_synch()
calls on Windows, so that a single one can be done once
the selectsave processing is overwith.

This was a little messy because an indicator had to flow
down through validate(), uptodate(), etc.

There shouldn't be any change in how things behave on
any non-Windows platforms.

sys/windows/windsys.c

@@ -491,7 +491,7 @@ nethack_exit(int code)
         /* use our custom version which works
            a little cleaner than the stdio one */
         windowprocs.win_nhgetch = windows_console_custom_nhgetch;
-    }
+    } else
 #endif
     if (getreturn_enabled) {
         raw_print("\n");
@@ -525,7 +525,8 @@ getreturn(const char *str)
     raw_print(buf);
     if (WINDOWPORT(tty))
         windows_console_custom_nhgetch();
-    wait_synch();
+    else
+        wait_synch();
     in_getreturn = FALSE;
     return;
 }