mod_auth_digest: Use siphash for the nonce hash where available:

* modules/aaa/mod_auth_digest.c (gen_nonce_hash): Use apr_siphash24_auth()
  where APU 1.6 or later provides it, keeping the SHA-1 hash otherwise.
  Take a pool for the message buffer, and length-prefix the realm.

* test/modules/aaa/test_002_nonce.py (test_digest_021): Take the length of
  the hash from the nonce rather than assuming SHA-1's.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
GitHub: closes #730


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1937716 13f79535-47bb-0310-9956-ffa450edef68