This patch provides the upcall mechanism that will be used for communicating
with the RPCSEC client user daemons.

It sets up a 'ramfs' style filesystem (rpc_pipefs) that is populated with
named pipes. Each time the kernel initializes a new NFS, lockd, statd or
portmapper client, a directory automatically gets set up in this fs.
The directory is initially only populated with a single file "info"
that provides information such as the server IP address, the port number
and the RPC service for the benefit of the user daemon.

When an RPCSEC_GSS mechanism needs to communicate with the daemon, it
is provided with a toolkit for setting up a named pipe in the same
directory. It can then perform upcalls/downcalls in order to talk to the
daemon in much the same way as is done by CODA.

The NFSv4 client will also need to use this same filesystem to communicate
with its user daemon in order to do name-to-uid/name-from-uid and
name-to-gid/name-from-gid translation.