cron(8): set the environment variables of the user and/or login class

Prior to processing environment variable set in the crontab file as those
should be of higher precedent, pull in the user or login class environment.

This is another supporting feature for allowing one to configure system-wide
settings that may affect both regular cron jobs as well as services.

This is the final part of D21481.

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