networkd: link - operstate is an enum, not a bitmask
networkd-wait-online: fix false positives when checking if a link is managed by networkd
networkd-wait-online: flush monitor events after processing

Otherwise the event will trigger immediately again.
timesyncd: only run when the system has a carrier on a network interface

As the operational state detection in sd-network is still too primitive, timesyncd
will likely try to connect a bit early, so the first attempt will fail.