MFC 260926:
Add support for displaying VPD for PCI devices via pciconf.
- Store the length of each read-only VPD value since not all values are
  guaranteed to be ASCII values (though most are).
- Add a new pciio ioctl to fetch VPD for a single PCI device.  The values
  are returned as a list of variable length records, one for the device
  name and each keyword.
- Add a new -V flag to pciconf's list mode which displays VPD data for
  each device.