ncurses 6.0 - patch 20150627

ncurses 6.0 - patch 20150627

+ modify configure script to remove deprecated ABI 5 symbols when
  building ABI 6.
+ add symbols _nc_Default_Field, _nc_Default_Form, _nc_has_mouse to
  map-files, but marked as deprecated so that they can easily be
  suppressed from ABI 6 builds (Debian #788610).
+ comment-out "screen.xterm" entry, and inherit screen.xterm-256color
  from xterm-new (report by Richard Birkett) -TD
+ modify read_entry.c to set the error-return to -1 if no terminal
  databases were found, as documented for setupterm.
+ add test_setupterm.c to demonstrate normal/error returns from the
  setupterm and restartterm functions.
+ amend cleanup change from 20110813 which removed redundant definition
  of ret_error, etc., from tinfo_driver.c, to account for the fact that
  it should return a bool rather than int (report/analysis by Johannes
  Schindelin).