Relay protocol: check string lengths

Ensure that string lengths are not longer than what the protocol
supports on both send and receive. Ensure that path lengths fit in the
local filename length limits on the receive side. Ensure that strings
exchanged in the relay protocol are zero-terminated.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>