Address nickm's comments at https://trac.torproject.org/projects/tor/ticket/933#comment:8

    1. Only allow '*.' in MapAddress expressions. Ignore '*ample.com' and '.example.com'.
       This has resulted in a slight refactoring of config_register_addressmaps.
    2. Add some more detail to the man page entry for AddressMap.
    3. Fix initialization of a pointer to NULL rather than 0.
    4. Update the unit tests to cater for the changes in 1 and test more explicitly for
       recursive mapping.