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/unix/hints/include/cross-pre2.370

@@ -235,8 +235,8 @@ ifeq "$(WANT_DEBUG)" "1"
 GDBEXE=$(dostargetexes)gdb.exe
 GDBBAT=NHGDB.BAT
 GDBCMDLINE=directory nhsrc/src nhsrc/include nhsrc/sys/msdos \
-		nhsrc/sys/share nhsrc/win/share nhsrc/win/curses \
-		nhsrc/win/tty nhsrc/util
+		nhsrc/sys/share nhsrc/win/curses \
+		nhsrc/win/tty
 else
 GDBEXE=
 GDBBAT=