libceph: amend cephx init_protocol() and build_request()

In msgr2, initial authentication happens with an exchange of msgr2
control frames -- MAuth message and struct ceph_mon_request_header
aren't used.  Make that optional.

Stop reporting cephx protocol as "x".  Use "cephx" instead.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>