Fix os_process_error on commonjs require

If a show function had a require statement outside of the scope of
function(doc) then the any requests to that show function failed
with {"error":"os_process_error","reason":"{exit_status,139}"}.

This patch adds the couch_args to the private field for any sub-context
that is created in evalcx, so that if evalcx is called for the new
context, couch_args is available.

To reproduce: https://gist.github.com/1603147