MFC r230453:

- Configurations in ipv6_prefix_IF should be recognized even if there is no
  ifconfig_IF_ipv6.

- DAD wait should be performed at once, not on a per-interface basis, if
  possible.  This fixes an issue that a system with a lot of IPv6-capable
  interfaces takes too long for booting.

Approved by:	re (glebius)
MFC 252576:
Don't perform the acpi_DeviceIsPresent() check for PCI-PCI bridges.  If
we are probing a PCI-PCI bridge it is because we found one by enumerating
the devices on a PCI bus, so the bridge is definitely present.  A few
BIOSes report incorrect status (_STA) for some bridges that claimed they
were not present when in fact they were.

While here, move this check earlier for Host-PCI bridges so attach fails
before doing any work that needs to be torn down.

PR:		kern/91594
Approved by:	re (marius)