Allow override of DUN settings

Allow override of system DUN settings by setting
persist.sys.dun.override
to one of the following values:
2 = not set, 0 = DUN not required, 1 = DUN required
If the prop is not set or is set to an invalid value the system setting
will be used.

Change-Id: I296f303a23351e4ab29898895abfcd313c747db9