RAID: Add new 'raid10_segtype_default' setting in lvm.conf
If '--mirrors/-m' and '--stripes/-i' are used together when creating
a logical volume, mirrors-over-stripes is currently chosen. The user
can override this by using the '--type raid10' option on creation.
However, we want a place where we can set the default behavior to
'raid10' explicitly - similar to the "mirror" and "raid1" tunable,
mirror_segtype_default.
A follow-on patch should use this new setting to change the default
from "mirror" to "raid10", as this is the preferred segment type.