boot time crash in if_detach_internal()

[Chris Torek]

>Not sure if the right answer is for drivers not to call ether_ifattach()
>until the point-of-no-failure (lots of drivers are wrong then) or
>initialize other parts earlier.

The other "obvious" method is to rearrange the sysinit priorities
(/sys/sys/kernel.h) so that all network domains are initialized
before invoking the device configuration code -- moving
SI_SUB_PROTO* before SI_CONFIGURE -- but presumably this idea was
tried and rejected earlier and hence the code in ether_ifattach
to check the same global variable.

Chris