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.

We need to add the GSSAPI CPPFLAGS to the CFLAGS for this module, as we
are not including the gssapi headers.
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.