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/options.c

@@ -151,7 +151,7 @@ static const struct Bool_Opt {
     { "legacy", &flags.legacy, TRUE, DISP_IN_GAME },
     { "lit_corridor", &flags.lit_corridor, FALSE, SET_IN_GAME },
     { "lootabc", &flags.lootabc, FALSE, SET_IN_GAME },
-#ifdef MAIL
+#ifdef MAIL_STRUCTURES
     { "mail", &flags.biff, TRUE, SET_IN_GAME },
 #else
     { "mail", (boolean *) 0, TRUE, SET_IN_FILE },