DedupeFind: AJAX getdupes method should search from the start on postcode

When people are entering postcodes it is not likely they would enter a string in the middle so searching should assume they are entering from the start of the string

DedupeFind.tpl uses js datatables to show the dupes rows. The filters above the list utilize datable's column search which also provides searching by regex but this has to be reflected serverside.
When done, we could allow searches by prefix for postal codes or other beneficous stuff.

For patch see PR8463
For use case see CRM-18693 / PR8464

Please review user manual search to see if an explanation is required about change to postal code search being a regex.

I have a concern with adding a regex without option to turn it off affecting performance significantly on large databases. Please ensure tests are done with >500k addresses to determine if this scales. If not, put in option to disable, but default to enabled.