mdev: implement hotplug support.

mdev: implement hotplug support.

Use DEVPATH, DEVNAME, MAJOR, MINOR, and SUBSYSTEM instead of
checking the current path and reading .../dev.
While we're here, probe for partitions in block devices.

This uses a very lame check for ACTION (which can be add, remove,
or change): if it is "remove", then unlink the device.