Add mbr pseudo metadata handler.

Add mbr pseudo metadata handler.

To support incorpating a new bare device into a collection of arrays -
one partition each - mdadm needs a modest understanding of partition
tables.
The main needs to be able to recognise a partition table on one device
and copy it onto another.

This will be done using pseudo metadata types 'mbr' and 'gpt'.

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