OPENAFS-SA-2018-001 butc: require authenticated connections with -localauth

OPENAFS-SA-2018-001 butc: require authenticated connections with -localauth

The butc -localauth option is available to use the cell-wide key to
authenticate to the vlserver and buserver, which in normal deployments
will require incoming connections to be authenticated as a superuser.
In such cases, the cell-wide key is also available for use in
authenticating incoming connections to the butc, which would otherwise
have been completely unauthenticated.

Because of the security hazards of allowing unauthenticaed inbound
RPCs, especially ones that manipulate backup information and are allowed
to initiate outboud RPCs authenticated as the superuser, default to
not allowing unauthenticated inbound RPCs at all.  Provide an opt-out
command-line argument for deployments that require this functionality
and have configured their network environment (firewall/etc.) appropriately.

Change-Id: Ia6349757a4c6d59d1853df1a844e210d32c14feb