net/bind916: Update to 9.6.18

This is a bugfix release.

- Fixed a bug that caused the NSEC salt to be changed for KASP zones on every
  startup.

- Signed, insecure delegation responses prepared by named either lacked the
  necessary NSEC records or contained duplicate NSEC records when both wildcard
  expansion and CNAME chaining were required to prepare the response. This has
  been fixed.

- Queries where the wildcard match contained the letter W failed to return the
  correct response as the W was mapped to \000.

- Checking of key-directory and dnssec-policy was broken. The checks failed to
  account for key-directory inheritance.

Full release notes:
https://bind9.readthedocs.io/en/v9_16/notes.html#notes-for-bind-9-16-18

(otis)