Warning message "the interface x is down" is incomplete. (GH#95)

<This was first reported as Github issue #95 (closed), by brubbel, on July 19, 2018>

(Note: This was tested on Kea 1.1.0, but I see that this issue is still present in the latest 1.4.0 version.) At startup, Kea may produce the following warning message: DHCPSRV_OPEN_SOCKET_FAIL failed to open socket: the interface x is down or has no usable IPv4 addresses configured

I checked interface x of course, but it was [UP] and had an IPv4 address assigned. It turns out (from reading the source) that Kea expects interface x also being in the [RUNNING] state.

I suggest to change the warning message into "has no usable IPv4 address or is missing flags [UP] and/or [RUNNING]."

As a related question: why does isc-dhcp allow and kea-dhcp does not allow starting a server on an interface without the IFF_RUNNING flag set? In any case, kea 1.1.0 does not work when the interface is not [RUNNING] when it was started, even when it becomes [RUNNING] afterwards. When the state goes from running->not running->running: all ok.