Introduce if_initialize and if_register as an alternative to if_attach

if_attach initializes an ifnet object and registers it to the system
(e.g., ifnet_list), however, if_attach doesn't complete the
initialization and the rest of it will be done by if_alloc_sadl
that is normally directly called by device drivers or called via
functions like ether_ifattach. So there is a race between
if_attach and if_alloc_sadl (A half-baked ifnet object may be
accessed, for example, via ioctl between them).

The aim of this fix is to register an initializing ifnet object
after completing its initializations. To this end, this fix
separates if_attach into an initialization part (if_initialize)
and a registration part (if_register) and call the latter after
if_alloc_sadl (ether_ifattach). So a typical usage of the two
new APIs is like this:

  if_initialize(ifp);  // was if_attach
  ether_ifattach(ifp, enaddr);
  if_register(ifp);

Nonetheless, changing every drivers to do so at once isn't
feasible. So we keep if_attach working as it used to be and
will change only some drivers that we need at this point.
Once we know the fix really works well, we'll change all
the others.

Some more information of the fix can be found here:
http://mail-index.netbsd.org/tech-kern/2014/12/10/msg018242.html

No objection on tech-kern and tech-net.
Add two new compat files for Xen on amd64
Don't pipeline any DMA writes to eMMC.

Fixes PR/49463.