relayd: Implement get_configuration relayd protocol command

relayd: Implement get_configuration relayd protocol command

Introduce a "GET_CONFIGURATION" command to the relayd protocol.
This allows checking whether the relay daemon allows the clear
command or not. A 64-bit set of flags are returned, and it can be
extended in the future with a variable length payload.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I33a03d09871f1c0c248d5514451c533ffd773979
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>