thin: initial --repair support for pools

thin: initial --repair support for pools

Initial basic support for repair.
It currently takes pool metadata spare volume, which
is used for recovery.  New spare is created if the volume
is successfuly repaired.

After the operation the previous _tmeta volume is moved
into  _tmeta%d volume and if everything is ok, this volume
could be removed.
New _tmeta needs to be pvmoved to proper place and also
converted to i.e. mirror if it should be mirrored.

Later version will try to automate some steps here.