When the state of the interface changes rapidly enough (usually due to rapid wireless association changes in my experience), there is a race where dhclient is in the process of exiting due to the link going down when the link coming up causes devd to try and start a new one.  This results is the link being up, but no dhclient running.
When the state of the interface changes rapidly enough (usually due to
rapid wireless association changes in my experience), there is a race
where dhclient is in the process of exiting due to the link going down
when the link coming up causes devd to try and start a new one.  This
results is the link being up, but no dhclient running.

Work around this race by checking a second time after a one second delay
before refusing to start a dhclient instance due to one already being
running.

MFC after:	1 week

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