Adds support for acl_plugin, and acl_opt_* options.
acl_opt_* requires some care as it relies on the internal behaviour of
cfg_load setting environment variables in a certain form.  However,
given that _all_ of the cfg_load infrastructure relies on that, we can
be pretty sure that it won't change in a way that will hurt us.

Originally reported as: https://github.com/openwrt/packages/pull/7434

Signed-off-by: Karl Palsson <karlp@etactica.com>