specl: propagate nested inprocess environments correctly.

* lib/specl/compat.lua (getfenv): New function to emulate Lua
getfenv in Lua 5.2.
* lib/specl/badargs.lua (diagnose): Propagate environment of
diagnose to calls of expect and examples for the rest of the
function.
* lib/specl/runner.lua (run_examples): Set environments of expect,
pending and examples to block environment.
(run_example): Likewise for badargs.diagnose.
* specs/badargs_spec.yaml (diagnose): Specify behaviour
accordingly.
* NEWS: Update.

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