Add kern.rpc.gss.client_max, to make it possible to bump it easily.
This can drastically lower the load on gssd(8) on large NFS servers.

Submitted by: Per Andersson <pa at chalmers dot se>
Reviewed by: rmacklem@
MFC after: 2 weeks
Sponsored by: Chalmers University of Technology
Differential Revision: https://reviews.freebsd.org/D18393