Introduce a new kernel-userspace interface that uses 128-bit file
identifiers instead of the previously used 96-bit fileids. We also
replacing the coda_creds structure with only the fsuid.

This new API has been used by for a couple of months now, people had to
patch their kernels whenever they want to run a current Coda release.
A new Kconfig option is added to fall back on the old API for older Coda
clients and other userspace filesystems that might use our protocol.