gssd: refactor update_client_list()

Split out the processing for a pipe to a separate routine.  The next
patch adds a new pipe to be processed.

Signed-off-by: Olga Kornievskaia <aglo@citi.umich.edu>
Signed-off-by: Kevin Coffman <kwc@citi.umich.edu>
Signed-off-by: Steve Dickson <steved@redhat.com>