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

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

Move to generating pccarddevs.h on the fly, both for the kernel and
the modules.

Also generate usbdevs.h automatically now, but a non-kernel file is
stopping that at the moment.

Add module infrastructure for 'if_ex'.