ncurses 6.1 - patch 20180908

+ document --with-pcre2 configure option in INSTALL.
+ improve workaround for special case in PutAttrChar() where a cell is
  marked as alternate-character set, to handle a case where the
  character in the cell does not correspond to any of the ASCII
  fallbacks (report by Leon Winter, cf: 20180505).
+ amend change to form library which attempted to avoid unnecessary
  update of cursor position in non-public fields, to simply disable
  output in this case (patch by Leon Winter, cf: 20180414).
+ improve check for LINE_MAX runtime limit, to accommodate broken
  implementations of sysconf().