dm table: add singleton feature

Introduce the concept of a singleton table which contains exactly one target.

If a target type sets the DM_TARGET_SINGLETON feature bit device-mapper
will ensure that any table that includes that target contains no others.

The thin provisioning pool target uses this.

Signed-off-by: Alasdair G Kergon <agk@redhat.com>

diff --cc drivers/md/dm-table.c
Simple merge

diff --cc include/linux/device-mapper.h
Simple merge