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.

include/windconf.h

@@ -268,7 +268,6 @@ extern int alternative_palette(char *);
 #endif
 
 #define nethack_enter(argc, argv) nethack_enter_windows()
-ATTRNORETURN extern void nethack_exit(int) NORETURN;
 extern boolean file_exists(const char *);
 extern boolean file_newer(const char *, const char *);
 #ifndef SYSTEM_H