doc: move DateTime functions back to their own LDoc section. (Fix #119)

* ext/posix/posix.c: Move Date and Time functions back to the
end of the implementation functions to avoid picking up non-
datetime related functions in the generated LDoc docs.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
slingshot: sync with upstream.

* slingshot: Update from latest upstream, particularly for an
empty table access fix in mkrockspecs.

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