separate MAIL functionality from MAIL-related structure inclusion

With 3.7+ aspirations of improving savefile interoperability between 32-bit
and 64-bit builds, as well as between platforms, it is better to not have
the underlying struct/array content be conditional.

This splits off some of the MAIL code into MAIL_STRUCTURES code. In theory,
since MAIL_STRUCTURES is unconditionally included, the macro could
just go away and leave that code unconditional, but this commit doesn't
go that far.

src/dogmove.c

@@ -432,7 +432,7 @@ int udist;
     } else {
         if ((obj = g.level.objects[omx][omy]) != 0
             && !index(nofetch, obj->oclass)
-#ifdef MAIL
+#ifdef MAIL_STRUCTURES
             && obj->otyp != SCR_MAIL
 #endif
             ) {