fix github issue #427 - unreadable mail

The change to make mail objects and monsters separate from mail
delivery (so that toggling the latter wouldn't invalidate save
and bones files) made it possible to wish for scrolls of mail,
find such in bones left by someone who did, or write such via
magic marker.  That was probably unintentional but I've left it
as-is.  The problem was that reading such scrolls issued a
warning:  "What weird effect is this?" because reading scrolls
of mail was only allowed when interacting with MAIL was enabled.

The issue suggested replacing #if MAIL with #if MAIL_STRUCTURES
in seffects(), and then insert #if MAIL in the part of reading
that deals with 'real' (or randomly faked for micros) mail.  I've
done both of those, and also added a couple of message variations
for the unreal cases.

Closes #427

doc/fixes37.0

@@ -1,4 +1,4 @@
-NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.400 $ $NHDT-Date: 1608749030 2020/12/23 18:43:50 $
+NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.401 $ $NHDT-Date: 1608846067 2020/12/24 21:41:07 $
 
 General Fixes and Modified Features
 -----------------------------------
@@ -445,6 +445,9 @@ a long worm with no visible segments (but one internal segment) might trigger
 	warning: tail 'segement' at <0,some_y>, worm at <mx,my> if teleported
 adding displacer beast inadvertently introduced a regression in swapping with
 	pets, allowing them to be pulled into water by hero on/over water
+splitting #if MAIL into #if MAIL_STRUCTURES and #if MAIL made it possible to
+	wish for and write scrolls of mail with MAIL disabled, but attempting
+	to read such a scroll issued impossible "What weird effect is this?"
 
 curses: 'msg_window' option wasn't functional for curses unless the binary
 	also included tty support