Refactor: lrmd: Rearrange lrmd_tls_dispatch.

• Get rid of the while loop over pcmk__remote_message_xml. There can only ever be one message in the buffer at a time, so this just makes things look more complicated than they really are.

• Invert a couple conditions, allowing code to be un-indented and the main thread of code to be easier to follow.

• Handle errors from pcmkremote_ready, pcmkread_remote_message, and pcmk__remote_message_xml with ifs instead of a switch. I think this is a little easier to follow.