service(8): set the environment of the "daemon" class before invoking

As mentioned in r357562, this gives the user a single place to configure
environment variables that need to be used for various services -- the
"daemon" class -- for, e.g., configuring a system-wide HTTP proxy.

This is a part of D21481.

Submitted by: Andrew Gierth <andrew_tao173.riddles.org.uk>