Make pkg use environment variables specified in pkg.conf

[moritz_schmi.tt]

Summary

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.

Test Plan

On a freshly installed FreeBSD system that needs to use a proxy for
accessing the internet, set http_proxy as shown in the example in
pkg.conf(5). Without the patch bootstrapping full pkg does not work;
with the patch, everything works as expected.