inprocess: new feature to run Lua programs inside Specl process.

* specs/inprocess_spec.yaml: New file. Specify capturing exit
status, and io streams while otherwise simulating running a
program in an external process.
* specs/specs.mk (specl_SPECS): Add specs/inprocess_spec.yaml.
* lib/specl/inprocess.lua: New file. Implement specifications.
* local.mk (dist_noinst_DATA): Add lib/specl/inprocess.lua.
* lib/specl/main.lua: Adjust to new calling convention.
* NEWS: Update.

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