networkd: link - stop clients when failing
networkd: link - avoid cycle when failing
networkd: link - log when starting clients fail
networkd: link - don't react to successfull link up

Avoid having two code-paths racing with eacother to do the same thing. The change
of flags will be detected in the normal way, so only use the link_up_handler
to detect if the 'up' failed and in that case fail the link.
networkd: link - support IFLA_OPERSTATE

This properly detects the state of the link based on both the link flags and the
operstate.

Moreover, always log state-changes even if we are not yet managing the link.
networkd: link - reduce default verbosity a bit