When we started calling if_findindex() from if_alloc() with an empty

When we started calling if_findindex() from if_alloc() with an empty
struct ifnet most of if_findindex() become a complex no-op.  Remove it
and replace it with a corrected version of the four line for loop it
devolved to plus some error handling.  This should probably be replaced
with subr_unit at some point.

Switch from checking ifaddr_byindex to ifnet_byindex when looking for
empty indexes.  Since we're doing this from if_alloc/if_free, we can
only be sure that ifnet_byindex will be correct.  This fixes panics when
loading the ef(4) module.  The panics were caused by the fact that
if_alloc was called four time before if_attach was called and thus
ifaddr_byindex was not set and the same unit was allocated again.  This
in turn caused the first if_attach to fail because the ifp was not the
one in ifnet_byindex(ifp->if_index).

Reported by:	"Wojciech A. Koszek" <dunstan at freebsd dot czest dot pl>
PR:		kern/84987
MFC After:	1 day