slingshot: sync with upstream for SPECL_ENV improvements.

* slingshot: Sync with upstream.
* specs/specs.mk: Simplify accordingly.
* specs/spec_helper.lua (package.path): Set according to slingshot
recommendations.
* bootstrap.conf: Require latest specl, and tidy accordingly.

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