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.

include/global.h

@@ -352,6 +352,14 @@ struct savefile_info {
 #define MAXMONNO 120 /* extinct monst after this number created */
 #define MHPMAX 500   /* maximum monster hp */
 
+/*
+ * Version 3.7.x has aspirations of portable file formats. We
+ * make a distinction between MAIL functionality and MAIL_STRUCTURES
+ * so that the underlying structures are consistent, whether MAIL is
+ * defined or not.
+ */
+#define MAIL_STRUCTURES
+
 /* PANICTRACE: Always defined for NH_DEVEL_STATUS != NH_STATUS_RELEASED
    but only for supported platforms. */
 #ifdef UNIX