dm table: share target argument parsing functions

Move multipath target argument parsing code into dm-table so other
targets can share it.

Change-Id: Ifb06ced86c10ceae89a82c846d79cbe1332f60d6
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>