As I hinted in #45380 [1], adding such an option to nsupdate might be
useful as well (if for nothing else then for consistency between tools).

rt45632 adds -4/-6 to both rndc and nsupdate, along with some further
related tweaks:

  - nsupdate should prefer ::1 over 127.0.0.1 when running in local-only
    mode (this is the part of rt45380 that has not yet been merged into
    master; commit 9c667d2243), if for nothing else then for consistency
    with how "server localhost" behaves,

  - command line options -4/-6 should be mutually exclusive, but delv
    and mdig did not enforce that,

  - all docs should present -4 and -6 as mutually exclusive options.

Please review.

[1] https://bugs.isc.org/Ticket/Display.html?id=45380#txn-488754