Refactor `user` into forked_user and forked_group

Refactor the single `user` attribute into two separate attributes (one
for user and one for group). This makes it more clear as to how you
would specify a group for forked child processes. A log resource was
also added to the definition to alert the end user if they only specified a
group and not a user. Unicorn does not support dropping group privileges
only and requires that a user also be specified.