dm table: share target argument parsing functions

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

Change-Id: I2502ec126cedd2f661c2a489f89cf096d0243460
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Merge "dm table: share target argument parsing functions" into du44