Mirror zone configuration tweaks and cleanups

Various discussions around mirror zones which happened after they were initially implemented (see #342 (closed), #375 (closed)/!475 (closed), Twitter poll) indicate that certain aspects of mirror zone configuration could use some tweaks:

• type secondary; mirror yes; should be replaced with type mirror;
• it should be clearly documented that:
  • mirror zones are supposed to replace the configuration example provided by RFC 7706 rather than augment it,
  • mirror zones only work the intended way if recursion is available in the view they are configured in,
  • by default, outgoing transfers of mirror zones are disabled.

Furthermore:

• the list of valid mirror zone options should be trimmed down,
• NOTIFY configuration for mirror zones is a bit too hacky and could use some love,
• we currently do not test whether mirror zones can be added and removed dynamically (using rndc).

The above concerns need to be addressed before we release BIND 9.14 and since configuration changes are involved, BIND 9.13.4 would be the more appropriate target.