While we currently have no need for messages larger than 65KB, we should design
the protocol to be future-proof, as we plan to keep at least to lowest protocol
layer stable.
To avoid any allocation issues, we currently keep the message size limit at
512KB.
As select() and finally the watcher may signal an FD even if it does not
actually have data, we must make a non-block read to avoid hanging in the
read callback.
Martin Willi