Dispatch-style protocol switching for transport

Dispatch-style protocol switching for transport

Switch to using per-transport-type functions when a socket that we're
using to communicate with a server becomes readable or writable, and add
them as pointers to the connection state.  The functions are passed the
name of the realm of the server being contacted, as we expect to need
this in the near future.

[nalin@redhat.com: replace macros with typedefs]
[nalin@redhat.com: compare transports with TCP_OR_UDP rather than with 0]

ticket: 7929