unistd: wrap pathconf minimally.

* specs/posix_spec.yaml (pathconf): Move from here...
* specs/posix_compat_spec.yaml (pathconf): ...to here.
* specs/posix_unistd_spec.yaml (pathconf): Specify a minimal
binding of pathconf(2).
* ext/posix/unistd.c (pathconf_map, Fpathconf, Kpathconf)
(Spathconf): Remove.
(pathconf): Simplify drastically.
(luaopen_posix_unistd): Add pathconf argument constants.
* lib/posix/compat.lua (pathconf): Reimplement legacy multi-
string or table i/o API over minimal C binding.
* NEWS: Update.

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