Implement SRXGK_GSSNegotiate()

In-kernel we must fail; the userspace implementation is already
in the tree in rxgk_gss.c.
Add rxgk_packet.c

Routines to apply and verify encryption and MICs to the data in
rx packets.  Backend to the rxgk_crypto framework for the actual
crypto operations.
Implement the rxgk server routines

Provide non-trivial implementations of the security class routines
used by the server, along with helpers as necessary.

In addition to the normal NewServerSecurityObject routine, provide
wrappers NewServiceSecObj and NewEphemeralService_SecObj that create
the security object, create an rx service for the key negotiation service
needed for clients to negotiate tokens useful with this server security
object, and register the getkey function used for the security object
as service-specific data to be used when encrypting tokens produced by
this rx service.  The Ephemeral variant creates a random token-encrypting
key for use by the service, so no getkey routine needs to be supplied
by the caller.

The identity supplied in a client's token is given as a list of
PrAuthNames; we assume that at most one name is supplied at present,
as the meaning of compound identities (and the use of compound
identities for keyed cache managers) is not fully specified yet.
Convert the PrAuthName to an rx_identity for caching in the server
connection state, as the rx_identity type is more compatible with
superuser checks on the connection.

Also provide an rxgk_GetServerInfo routine which extracts the cached
identity, for use in libauth when making superuser checks.
Implement rxgk client security object routines

Also add rxgk_NegotiateSecurityObject, a routine that creates
an rxnull connection to a target server, obtains a token using
the GSSNegotiate RPC on that connection, and uses the resulting
token to make an rxgk connection to that same server.
BuildServerSecurityObjects allocate for rxgk

Don't do anything with it for now, just leave a place in the
array.
Let superuser identities be superusers

We have a special rx_identity_kind for superusers, let it actually
be useful for something.
Add rxgk support to userok
DO NOT MERGE

A snapshot of the pthread-bos bits, all squashed together.
Done so that subsequent commits adding rxgk support to bos/bosserver
can be pushed to gerrit without using a merge commit or adding a
fake dependency chain to one of the patch series.
Register bosserver for rxgk

Add the server security object, using an ephemeral token-encrypting
key.  We rely on the local hostname being correctly set (and lowercase!)
to match the afs3-bos hostbased service acceptor identity.

Since we are adding a new service, set the number of processes that
will handle that service.
bos: add support for rxgk

While rxgk is experimental, add a new argument -rxgk to attempt
the use of rxgk.  -localauth is supported by using the acceptor
credentials as an initiator and self-negotiating.

afsconf_PickClientSecObj doesn't know about rxgk yet, so we need
to have our own separate control flow for the rxgk case.  This
will be fixed eventually, but it's not clear what the right API
for PickClientSecObj is, yet.
local build fix