dig.SPF fixes

This pull request does a few things:

* Simplifies ``__virtual__`` function
* Use ``socket.inet_pton()`` to validate IP addresses, instead of error-prone regular expressions
* Add aliases for functions in ``dig`` execution module, so that ``dig.spf``, ``dig.a``, etc. also work.
* Fix #10367 by recursing ``redirect`` and ``include`` entries.
* Adds a new test for ``redirect=somedomain`` SPF records