Use setfsuid() rather than seteuid() while creating contexts

Signed-off-by: Kevin Coffman <kwc@citi.umich.edu>
As suggested by Olaf Kirch <okir@suse.de>, use setfsuid() rather than
seteuid() when creating a gss context.  This prevents users from using
credentials that do not belong to them, while also preventing them from
doing things like killing, renicing, or changing the priority of the
gssd process while it is processing the context creation.