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To allow some tests to quickly check if some group membership handling
are correctly done, the tests need to run as a proc belonging to multiple
groups.
su would normally only allow setting uid=gid=something.

This changes adds setting the gid separately from the uid and setting the
groups:
  su [uid[,gid[,group1]...]] [cmd]]
E.g.
 su
 su shell ls /data
 su 1000,shell,net_bw_acct,net_bw_stats id
will return
 uid=1000(system) gid=2000(shell) groups=3006(net_bw_stats),3007(net_bw_acct)

Change-Id: I713454bf14c3bc3ece93aec3729bda102542d1b1