Add a new device flag: DF_ATTACHED_ONCE

This flag is set once the device has been successfully attached. When
set, it inhibits devmatch from trying to match the device. This in
turn allows kldunload to work as expected. Prior to the change, the
driver would immediately reload because devmatch had no notion that
the driver had once been attached, and therefore shouldn't participate
in further matching.

Differential Revision: https://reviews.freebsd.org/D16735