Make pkg(7) use environment variables specified in pkg.conf
Modify /usr/sbin/pkg to use environment variables specified in pkg.conf.
This allows control over underlying libraries like fetch(3), which can
be configured by setting HTTP_PROXY.
Differential Revision: https://reviews.freebsd.org/D29820
(cherry picked from commit e869d3c60147bbb226b5ad97d2ef73391aeebafa)