specl: upgrade specs for Specl 6.

* configure.ac (SPECL_MIN): Bump to 6.
* specs/posix_spec.yaml: Require specl.shell for spawn API.
(cmd_output): Use spawn instead of undocumented `specl.cmdpipe`.
(describe ctermid): Use `any_of` to allow alternatives.
(describe getenv): Skip volatile environment vars that are likely
to change in a subshell.

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