posix: use strlcpy instead of broken strncpy.

* ext/curses/strlcpy.c: Move from here...
* ext/include/strlcpy.c: ...to here.
(strlcpy): Declare static to prevent symbol clashes.
* local.mk (ext_curses_curses_c_la_CPPFLAGS): Remove. strlcpy is
now in existing header directory.
* build-aux/sanity-cfg.mk: Adjust strlcpy path references.
* ext/posix/posix.c: Include strlcpy.c.
(sockaddr_from_lua, Psetsockopt): Use strlcpy to avoid security
and safety problems with strncpy.

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