ncurses 6.1 - patch 20180519

ncurses 6.1 - patch 20180519

+ formatting fixes for manpages, regenerate man-html documentation.
+ trim spurious whitespace from tmux in 2018-02-24 changes;
  fix some inconsistencies in/between tmux- and iterm2-entries for SGR
  (report by C Anthony Risinger)
+ improve iterm2 using some xterm features which it has adapted -TD
+ add check in pair_content() to handle the case where caller asks
  for an uninitialized pair (Debian #898658).