Block drivers: make IOCTL request unsigned long

Block drivers: make IOCTL request unsigned long

The block driver protocol and libblockdriver's bdr_ioctl hook are
changed, as well as the users of this hook. Other parts of the system
are expected to change accordingly eventually, since the ioctl(2)
prototype has been aligned with NetBSD's.

Change-Id: Ide46245b22cfa89ed267a38088fb0ab7696eba92