specs: move stdlib function specs to their own spec-file.

* specs/posix_spec.yaml (abort, getenv, grantpt, mkdtemp, mkstemp)
(openpt, ptsname, realpath, unlockpt, setenv): Move from here...
* specs/posix_stdlib_spec.yaml (abort, getenv, grantpt, mkdtemp)
(mkstemp, openpt, ptsname, realpath, unlockpt, setenv): New file.
...to here.
* specs/specs.mk (specl_SPECS): Add specs/posix_stdlib_spec.yaml.

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