unistd: wrap sysconf minimally.

* specs/posix_spec.yaml (sysconf): Move from here...
* specs/posix_compat_spec.yaml (sysconf): ...to here.
* specs/posix_unist_spec.yaml (sysconf): Specify minimal
binding of sysconf(2).
* ext/posix/unistd.c (sysconf_map, Fsysconf, Ksysconf, Ssysconf):
Remove.
(sysconf): Simplify drastically.
(luaopen_posix_unistd): Add sysconf argument constants.
* lib/posix/compat.lua (sysconf): Reimplement legacy multi-string
or table i/o API over minimal C binding.
* NEWS: Update.

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