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

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

Add a ``digi'' driver.

This driver supports PCI Xr-based and ISA Xem Digiboard cards.
dgm will go away soon if there are no problems reported. For now,
configuring dgm into your kernel warns that you should be using
digi. This driver is probably close to supporting Xi, Xe and Xeve
cards, but I wouldn't expect them to work properly (hardware
donations welcome).

The digi_* pseudo-drivers are not drivers themselves but contain
the BIOS and FEP/OS binaries for various digiboard cards and are
auto-loaded and auto-unloaded by the digi driver at initialisation
time. They *may* be configured into the kernel, but waste a lot
of space if they are. They're intended to be left as modules.

The digictl program is (mainly) used to re-initialise cards that
have external port modules attached such as the PC/Xem.