curses: remove separate curses doc generation.

* build-aux/make_lcurses_doc.pl: Remove.
* local.mk (DIST_DOC_DATA, EXTRA_DIST): Adjust accordingly.
(classesdir): Location for ldoc class html files.
(dist_classes_DATA): Add new generated curses class docs.
* ext/posix/curses.c: Rewrite equivalent docs here.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>