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/bones.c

@@ -135,7 +135,7 @@ boolean restore;
 
             if (otmp->otyp == SLIME_MOLD) {
                 goodfruit(otmp->spe);
-#ifdef MAIL
+#ifdef MAIL_STRUCTURES
             } else if (otmp->otyp == SCR_MAIL) {
                 /* 0: delivered in-game via external event;
                    1: from bones or wishing; 2: written with marker */