some tty updates

Add a note about NO_TERMS to include/wintty.h for clarity.

Rename tty_startup and tty_shutdown to term_startup() and
term_shutdown(). They are found in termcap.c for !NO_TERMS
like most of the other term_ routines, as well as having
versions for several of the NO_TERMS platforms. They aren't
part of the tty_interface called from the core. The tty
implementation does call and rely on them.

Remove some conditional #ifdef's around term_shutdown()
(formerly tty_shutdown()) and just ensure that all the
tty platforms have an implementation that they can link
with, even if it is just a stub presently.

Put the protype for nethack_exit in extern.h to reduce
maintenance to a single spot, and remove it from other
locations. A warning in the msdos cross-compile led to
this change.

sys/msdos/vidvesa.c

@@ -603,7 +603,7 @@ vesa_tty_end_screen(void)
 }
 
 void
-vesa_tty_startup(int *wid, int *hgt)
+vesa_term_startup(int *wid, int *hgt)
 {
     /* code to sense display adapter is required here - MJA */
 
@@ -2042,6 +2042,16 @@ vesa_SetSoftPalette(const struct Pixel *palette)
     return TRUE;
 }
 
+void
+vesa_hide_cursor(void)
+{
+}
+
+void
+vesa_show_cursor(void)
+{
+}
+
 #ifdef POSITIONBAR
 
 #define PBAR_ROW (LI - 4)