curses: use _helpers.c macros.

* ext/posix/curses.c (LCURSES_SPLICE, LCURSES_STR, LCURSES_STR_1):
Remove. Adjust all callers to use LPOSIX equivalents from
ext/include/_helpers.c.
(curseslib, windowlib): Use LPOSIX_FUNC to declare table entries
rather than custom MENTRY macros.

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