Break out allocation of new ifindex values from if_alloc() and if_vmove(), and centralize in a single function ifindex_alloc().  Assert the IFNET_WLOCK, and add missing IFNET_WLOCK in if_alloc().  This does not close all known races in this code.
Break out allocation of new ifindex values from if_alloc() and if_vmove(),
and centralize in a single function ifindex_alloc().  Assert the
IFNET_WLOCK, and add missing IFNET_WLOCK in if_alloc().  This does not
close all known races in this code.

Reviewed by:	bz
MFC after:	3 days

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