Add fbd -- Faulty Block Device driver

This driver can be loaded as an overlay on top of a real block
device, and can then be used to generate block-level failures for
certain transfer requests. Specifically, a rule-based system allows
the user to introduce (overt and silent) data corruption and errors.

It exposes itself through /dev/fbd, and a file system can be mounted
on top of it. The new fbdctl(8) tool can be used to control the
driver; see ``man fbdctl'' for details. It also comes with a test
set, located in test/fbdtest.

docs/UPDATING

@@ -1,3 +1,8 @@
+20111212:
+	After a successful "make world", issue the following commands:
+	# cd /dev
+	# MAKEDEV fbd
+
 20111109:
 	Switch to NetBSD passwd system.