[9.18] Resolve "Restore the ability to read old HMAC-MD5 key pair files."
requested to merge 4154-restore-the-ability-to-read-old-hmac-md5-key-pair-files-bind-9.18 into bind-9.18
Backport of MR !8041 (merged)
Closes #3668 (closed), #4154 (closed)