rework TTY_PERM_INVENT; update window port interface

Change the inner workings of the experimental TTY_PERM_INVENT.

Switch to delivering the content to tty for the experimental perm_invent
via the existing window port interface (start_menu(), add_menu(), end_menu).

This also adds a new window port interface call ctrl_nhwindow() for
delivering information to the window port, and/or obtaining specific
information from the window port. The information and requests can
be extended as required. To be documented later once the changes settle
down.

Due to the intrusive nature of these changes and the possibility of
some bugs in the new code, I'm going to leave TTY_PERM_INVENT commented
out in the repository for a day or two.  Anyone wishing to test it out
can do so by uncommenting TTY_PERM_INVENT in config.h.

src/mdlib.c

@@ -96,13 +96,13 @@ char optbuf[COLBUFSZ];
 static struct version_info version;
 static const char opt_indent[] = "    ";
 
-struct win_info {
+struct win_information {
     const char *id, /* DEFAULT_WINDOW_SYS string */
         *name;      /* description, often same as id */
     boolean valid;
 };
 
-static struct win_info window_opts[] = {
+static struct win_information window_opts[] = {
 #ifdef TTY_GRAPHICS
     { "tty",
       /* testing 'USE_TILES' here would bring confusion because it could