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

src/lib/krb5/os/changepw.c

@@ -261,9 +261,9 @@ change_set_password(krb5_context context,
         callback_info.pfn_cleanup = kpasswd_sendto_msg_cleanup;
         krb5_free_data_contents(callback_ctx.context, &chpw_rep);
 
-        code = k5_sendto(callback_ctx.context, NULL, &sl, strategy,
-                         &callback_info, &chpw_rep, ss2sa(&remote_addr),
-                         &addrlen, NULL, NULL, NULL);
+        code = k5_sendto(callback_ctx.context, NULL, &creds->server->realm,
+                         &sl, strategy, &callback_info, &chpw_rep,
+                         ss2sa(&remote_addr), &addrlen, NULL, NULL, NULL);
         if (code) {
             /*
              * Here we may want to switch to TCP on some errors.

src/lib/krb5/os/os-proto.h

@@ -115,6 +115,7 @@ int _krb5_use_dns_kdc (krb5_context);
 int _krb5_conf_boolean (const char *);
 
 krb5_error_code k5_sendto(krb5_context context, const krb5_data *message,
+                          const krb5_data *realm,
                           const struct serverlist *addrs,
                           k5_transport_strategy strategy,
                           struct sendto_callback_info *callback_info,