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/mkobj.c

@@ -980,7 +980,7 @@ boolean artif;
             break;
         case POTION_CLASS: /* note: potions get some additional init below */
         case SCROLL_CLASS:
-#ifdef MAIL
+#ifdef MAIL_STRUCTURES
             if (otmp->otyp != SCR_MAIL)
 #endif
                 blessorcurse(otmp, 4);