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.

win/win32/NetHackW.c

@@ -53,7 +53,6 @@ Version     _WIN_32IE   Platform/IE
 /*#define COMCTL_URL
  * "http://www.microsoft.com/msdownload/ieplatform/ie/comctrlx86.asp"*/
 
-ATTRNORETURN extern void nethack_exit(int) NORETURN;
 static TCHAR *_get_cmd_arg(TCHAR *pCmdLine);
 static HRESULT GetComCtlVersion(LPDWORD pdwMajor, LPDWORD pdwMinor);
 BOOL WINAPI