hangup paranoia (trunk only)

hangup paranoia (trunk only)

     If nhwindows_hangup() were to get called a second time, it would have
saved hup_exit_nhwindows() as the previous interface's shutdown routine.
Then if/when exit_nhwindows() gets called, hup_exit_nhwindows() would
have called itself with uncontrolled recursion.  So guard against that by
preventing the hangup shutdown routine from ever being saved as previous
interface shutdown routine.

     Also, avoid the exit_windows macro when using it as a pointer rather
than for a function call, since pre-ANSI compilers might be confused by
its expansion.