svcgssd: Adding a <-p principal> flag

Allow the principal that is used to get the machines creds definable
on the command like with the new '-p <principal>'. This is useful
in cluster environments.

Signed-off-by:  Eberhard Kuemmerle <E.Kuemmerle@fz-juelich.de>
Signed-off-by: Steve Dickson <steved@redhat.com>