doc: fix LDoc manual page custom_see_handler pattern.

* ext/posix/config.ld (custom_see_handler): Add a missing closing
`%)`.
(upat): Pull the format string up per LDoc documentation.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
doc: add missing LDoc function tag.

* ext/posix/posix.c (strptime): Add missing @function tag.

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

* slingshot: Sync with upstream.
* bootstrap: Update from slingshot.
* bootstrap.slingshot: Remove, no longer required.
* bootstrap.conf: Remove bootstrap.slingshot source boilerplate.
(luaposix_force_readme): Remove. Automated by slingshot now.
* .gitignore: Update.
* .travis.yml: Until github.com/stevedonovan/LDoc catches up with
require fixes for luaposix, use gvvaughan/LDoc on Travis.

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