ncurses 6.0 - patch 20171007

ncurses 6.0 - patch 20171007

+ modify "-T" option of clear and tput to call use_tioctl() to obtain
  the operating system's notion of the screensize if possible.
+ review/repair some exit-codes for tput, making usage-message exit
  with 2 rather than 1, and a failure to open terminal 4+errno.
+ amend check in tput, tabs and clear to allow those to use the
  database-only features in cron if a -T option gives a suitable
  terminal name (report by Lauri Tirkkonen).
+ correct an ifdef in test/ncurses.c for systems with soft-keys but
  not slk_color().
+ regenerate man-html documentation.