Fixed up the user/group contexts when using authenticated pipes.

Fixed up the user/group contexts when using authenticated pipes.
Added a become_root()/unbecome_root() (push/pop security context)
around the initgroups() call to ensure it would succeed. Hmmm - I
wonder if this call being done as non-root might explain any "group access"
bugs we've had in the past....
Jeremy.
(This used to be commit 06a65972e872f37d88b84f22ea714feebd38f6c0)