Systemd integration for starting newly-degraded arrays.

Systemd integration for starting newly-degraded arrays.

Normally "mdadm -I" will not start an array if it has reason to
expect further devices.
This means that if a device is removed while the host is shut down,
"mdadm -I" will never start the device.

If  the array is know to the host, it make sense to start the array
anyway after a reasonable timeout.

This patch adds systemd/udev infrastructure so that 30 seconds after
a known array first becomes able to be assembled as a degraded array,
the array will be assembled even if more devices are still expected.

Signed-off-by: NeilBrown <neilb@suse.de>