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/vidvga.c

@@ -297,7 +297,7 @@ vga_tty_end_screen(void)
 }
 
 void
-vga_tty_startup(int *wid, int *hgt)
+vga_term_startup(int *wid, int *hgt)
 {
     /* code to sense display adapter is required here - MJA */
 
@@ -1159,6 +1159,16 @@ vga_SetPalette(const struct Pixel *p)
     }
 }
 
+void
+vga_hide_cursor(void)
+{
+}
+
+void
+vga_show_cursor(void)
+{
+}
+
 /*static unsigned char colorbits[]={0x01,0x02,0x04,0x08}; */ /* wrong */
 static unsigned char colorbits[] = { 0x08, 0x04, 0x02, 0x01 };