maint: retain backwards compatibility with v32 posix.stat.

We now have `posix.sys.stat` for thin bindings of stat and lstat,
so maintain backwards compatibility with v32 in `posix.stat`.
* specs/posix_compat_spec.yaml (lstat): Remove.
(stat): Adjust to expect lstat under the hood.
* specs/posix_spec.yaml: Adjust stat/lstat clients.
* lib/posix/compat.lua (lstat): Remove.
(stat): Adjust to use lstat under the hood.
* NEWS: Update.

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