gssd: handle new client upcall

gssd: handle new client upcall

Add support for handling the new client-side upcall.  The kernel,
beginning with 2.6.29, will attempt to use a new pipe, "gssd",
which can be used for upcalls for all gss mechanisms.

The new upcall is text-based with an <attribute>=<value> format.
Attribute/value pairs are separated by a space, and terminated
with a new-line character.

The intial version has two required attributes,
mech=<gss_mechanism_name> and uid=<user's_UID_number>, and two
optional attributes, target=<gss_target_name> and service=<value>.

Future kernels may add new attribute/value pairs.

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>