Manage: simplify `rdev` handling in Manage_subdevs.

The only use 'struct stat stb' to get the 'rdev', and sometimes
we don't even use 'stat'.
So make 'rdev' a stand-alone variable, and only declare stb'
when we actually need it.

Signed-off-by: NeilBrown <neilb@suse.de>
util: split get_maj_min() out from dev_open()

This allows other code to parse "8:3" style device names.

Signed-off-by: NeilBrown <neilb@suse.de>
Manage: fix removal of non-existent devices.

"--remove detached" and others stopped working a while
back when I refactored some code.
For 'remove' and  'fail', the device may not exist so
if it is  "MM:mm", (e.g. added by "detached"), just parse
out the numbers.

Reported-by: Killian De Volder <killian.de.volder@megasoft.be>
Signed-off-by: NeilBrown <neilb@suse.de>
mdadm: document some more magic environment variables.

Others are mostly for developers.

Signed-off-by: NeilBrown <neilb@suse.de>
Monitor: Stop monitoring devices that have disappeared.

If we are only monitoring a device because we found it in
/proc/mdstat, and it has been gone for 5 checks, forget
about it completely.

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