Rename `AuthMethod` to `SecurityMechanism`.

Since applicable security mechanisms are not just for authentication,
but also for confidentiality, e.g. the CURVE mechanism (not yet
supported).
s/createTransportEx/createTransportExposeInternals/.

Mirror function with same name in n-t-tcp.
s/ZMQTransport/TransportInternals/.

Mirror datatype with same name in n-t-tcp.
extraneous parens.