Add IFNET_HOLD reserved pointer value for the ifindex ifnet array,
which allows an index to be reserved for an ifnet without making
the ifnet available for management operations. Use this in if_alloc()
while the ifnet lock is released between initial index allocation and
completion of ifnet initialization.

Add ifindex_free() to centralize the implementation of releasing an
ifindex value. Use in if_free() and if_vmove(), as well as when
releasing a held index in if_alloc().

Reviewed by: bz
MFC after: 3 days