Add ability to override dialstring variables for destinations using mod_db entries

[markuslindenberg]

To help debugging transmissions to certain destinations, dialstring variables that should be applied only to faxes for the given destination can be added to FreeSWITCH's db:

db insert/override-4711/fax_verbose/true

Realm must be a string starting with override- and the exact destination number.

[denzs]

Tested successfully