flua: add freebsd.kenv, kenv(2) bindings

Add bindings for kenv(2) right now only get() has been created
it allows do dump into a key/value table the kernel environement if
no argument is passed, or it returns the value associated to the
provided key.

Reviewed by: imp, kevans, markj
Accepted by: imp, kevans
Differential Revision: https://reviews.freebsd.org/D46654