dig.SPF fixes

**NOTE: This breaks tests.unit.modules.DigTestCase.test_spf(), because this test was incorrect since redirects still needed to be recursed. Do not merge until I have fixed the tests as well. I will update the comments when it is ready to merge.**

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.