Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

In pci_cfgregread() and pci_cfgregwrite(), multiplex the domain and
bus number into the bus argument. The bus number occupies the least
significant 8 bits. The PCI domain occupies the most significant 24
bits.

On the Altix 350, the PCI domain is a required parameter, but
changing the prototype of the pci_cfgreg*() functions to include a
separate domain argument has wide-spread consequences across the
supported architectures. We'd be changing a known interface.

Multiplexing is an acceptable kluge to give us what we need with
manageable impact. Note that the PCI bus number fits in 8 bits,
so the multiplexing of the domain is a backward compatible change.

Disable interrupts when calling into SAL for PCI configuration
cycles. This serves 2 purposes:
1. It prevents preemption and CPU migration while running SAL code.
2. It reduces the chance of stack overflows: we're supposed to enter
SAL with at least 16KB of either memory- or register stack space,
which we can't do without switching to a different stack.

In pci_cfgregread() and pci_cfgregwrite(), validate the arguments and check
that the alignment matches the width of the read or write.

The encoding for the bus being passed to SAL was completely wrong.

Start hooking up devices.