ncurses 6.0 - patch 20170429

ncurses 6.0 - patch 20170429

+ corrected a case where $with_gpm was set to "maybe" after CF_WITH_GPM,
  overlooked in 20160528 fixes (report by Alexandre Bury).
+ improve a couple of test-program's help-messages.
+ corrected loop in rain.c from 20170415 changes.
+ modify winnstr and winchnstr to return error if the output pointer is
  null, as well as adding a null pointer check of the window pointer
  for better compatibility with other implementations.
+ improve discussion of NetBSD curses in scr_dump.5
+ modify LIMIT_TYPED macro in new_pair.h to avoid changing sign of the
  value to be limited (reports by Darby Payne, Rob Boudreau).
+ update config.guess, config.sub from
	http://git.savannah.gnu.org/cgit/config.git