Fix relative numerical addressing (addr,+N).

As a bonus the patch untangles a bit the logic and makes the code
easier to grasp.

PR:		192108
MFC after:	1 week
When restoring a UFS dump onto a ZFS filesystem, an assertion in
restore was failing because ZFS was reporting a blocksize that was
not a multiple of 1024.  Replace restore's failed assertion with
code that writes restored files in a blocksize that works for
restore (a multiple of 1024) despite being non-optimal for ZFS.

Submitted by: Dmitry Morozovsky
Tested by:    Dmitry Morozovsky
MFC after:    1 week
Add new rule to source address selection algorithm. It prefers address
with better virtual status. Use ifa_preferred() to choose better address.

PR:		187341
Tested by:	des
MFC after:	1 week