maint: reinstate specl package.path workaround for luarocks bug.

Now that we're supporting LuaRocks' `init.lua suffixed filenames
get installed to an init installation directory` bug again, we
must adjust Specl's in-tree package.path to accommodate.
* specs/spec_helper.lua.in (package.path): Add "lib/?/init.lua".
* local.mk (std_path): Likewise

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