env(1): grow -L user/class and -U user/class options

This allows one to set the environment of the specified user either from
login.conf alone (-L) or both login.conf and ~/.login_conf if present (-U).

This is a supporting feature to allow service(8) to pull in the environment
of the "daemon" class before invoking the rc script.

This is a part of D21481.

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