dm table: share target argument parsing functions

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

Change-Id: Icd51cc97b1d8c71f613c10c54493e087d5f99b2f
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>