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.

etc/system.conf

@@ -540,3 +540,13 @@ service vbox
 	;
 	uid	0;
 };
+
+service fbd
+{
+	ipc
+		SYSTEM VFS RS DS VM
+		ahci
+		at_wini
+		bios_wini
+	;
+};