[PATCH] Add MSI support to puc(9)

[Ryan Stone]

The Exar XR17V358 is a PCIe device and it supports a single MSI
interrupt.  This patch will make puc(9) use an MSI interrupt on
devices that declare that they support it:

http://people.freebsd.org/~rstone/patches/puc_msi.diff

This patch may be overly paranoid; I was worried that it's wasn't
guaranteed that I could always call pci_alloc_msi() (forgetting that
the P in puc stands for PCI) so I added a new puc_cfg_cmd that
individual device config methods could implement to advertise support
rather than depending on pci_alloc_msi() to behave sanely.

I have tested the patch on both a XR17V358 and a XR17V258 (which is a
legacy PCI device that does not support PCI)