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/exper.c

@@ -128,7 +128,7 @@ register int nk;
     if (mtmp->m_lev > 8)
         tmp += 50;
 
-#ifdef MAIL
+#ifdef MAIL_STRUCTURES
     /* Mail daemons put up no fight. */
     if (mtmp->data == &mons[PM_MAIL_DAEMON])
         tmp = 1;