Enhance raid4 support: --assemble and --monitor wasn't quite happy with it.

Enhance raid4 support: --assemble and --monitor wasn't quite happy with it.

From: Doug Ledford <dledford@redhat.com>

This one actually does a couple things.  Mainly related to raid4, but
kinda touches other raid levels some.

When creating a raid4 array, treat it like a raid5 array in that we
create it in degraded mode by default and add the last disk as a spare.
Besides speeding things up, this has a second effect that it makes mdadm
more consistent.  In order to create a degraded raid5 array, you need
only passing missing as one of the devices.  For a degraded raid4 array,
prior to this patch, you must pass assume-clean or else it refuses to
create the array.  Even force won't make it work without assume-clean.
With the patch, raid4 behaves identical to raid5.

Separate from that, the monitor functionality completely ignores raid4
arrays.  That seems to stem from the code that checks to see if the
array is part of a long list of types.  It seems easier to check which
array types *aren't* redundant instead of listing the ones that are
redundant and missing some of them.  This makes the monitor service
actually watch raid4 arrays.