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/monmove.c

@@ -925,7 +925,7 @@ register int after;
         mmoved = 0;
     }
 
-#ifdef MAIL
+#ifdef MAIL_STRUCTURES
     if (ptr == &mons[PM_MAIL_DAEMON]) {
         if (!Deaf && canseemon(mtmp))
             verbalize("I'm late!");