specl: remove posix libraries from cache after use.

* lib/specl/util.lua: After saving references to the luaposix
functions this file uses, remove cached posix libraries in case
an example specl runs changes package.cpath and wants to check
behaviours of another luaposix installed there.
(have_posix): Export.
* lib/specl/main.lua: Import util.have_posix.
(process_args): Use have_posix rather than the cached table to
determine whether posix.files was available.
* NEWS: Update.

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