Add support for m-way to n-way up-convert in RAID1 (no linear to n-way yet)

Add support for m-way to n-way up-convert in RAID1 (no linear to n-way yet)

This patch adds the ability to upconvert a raid1 array - say from 2-way to
3-way.  It does not yet support upconverting linear to n-way.

The 'raid' device-mapper target allows for individual components (images) of
an array to be specified for rebuild.  This mechanism is used when adding
new images to the array so that the new images can be resync'ed while the
rest of the images in the array can remain 'in-sync'.  (There is no
mirror-on-mirror layering required.)