auth/cephx: add authorizer challenge

auth/cephx: add authorizer challenge

Allow the accepting side of a connection to reject an initial authorizer
with a random challenge.  The connecting side then has to respond with an
updated authorizer proving they are able to decrypt the service's challenge
and that the new authorizer was produced for this specific connection
instance.

The accepting side requires this challenge and response unconditionally
if the client side advertises they have the feature bit.  Servers wishing
to require this improved level of authentication simply have to require
the appropriate feature.

Signed-off-by: Sage Weil <sage@redhat.com>