device_disable(9) on FreeBSD

The text 6f8878c0d571, as FreeBSD 4.3-RELEASE, FreeBSD 4.2-RELEASE, FreeBSD 4.1.1-RELEASE, FreeBSD 4.1-RELEASE, FreeBSD 4.0-RELEASE shipped it.

manipulate device enabled flag

device_disable(9) on FreeBSD now