Make Worker#user support different process primary group and log file group
Previously, Unicorn always used the process's primary group as the
the group of the log file. However, there are reasons to use a
separate group for the log files, such as when you have many
applications where each application uses it's own user and primary
group, but you want to be able to have a user read the log files
for all applications. Some operating systems have a fairly small
limit on the number of groups per user, and it may not be feasible
to have a user be in the primary group for all applications.
a primary group