While FIDO/U2F keys were already supported by the generic uhid(4)
driver, this driver adds the first step to tighten the security of
FIDO/U2F access.  Specifically, users don't need read/write access to
all USB/HID devices anymore and the driver also improves integration
with pledge(2) and unveil(2): It is pledge-friendly because it doesn't
require any ioctls to discover the device and unveil-friendly because
it uses a single /dev/fido/* directory for its device nodes.
It also allows to support FIDO/U2F in firefox without further
weakening the "sandbox" of the browser.  Firefox does not have a
proper privsep design and many operations, such as U2F access, are
handled directly by the main process.  This means that the browser's
"fat" main process needs direct read/write access to all USB HID
devices, at least on other operating systems.  With fido(4) we can
support security keys in Firefox under OpenBSD without such a
compromise.
With this change, libfido2 stops using the ioctl to query the device
vendor/product and just assumes "OpenBSD" "fido(4)" instead.  The
ioctl is still supported but there was no benefit in obtaining the
vendor product or name; it also allows to use libfido2 under pledge.
With feedback from deraadt@ and many others
OK kettenis@ djm@ and jmc@ for the manpage bits