*) mod_md: when MDMessageCmd for a 'challenge-setup:<type>:<dnsname>'

  *) mod_md: when MDMessageCmd for a 'challenge-setup:<type>:<dnsname>'
     fails (!= 0 exit), the renewal process is aborted and an error is
     reported for the MDomain. This provides scripts that distribute
     information in a cluster to abort early with bothering an ACME
     server to validate a dns name that will not work. The common
     retry logic will make another attempt in the future, as with
     other failures.
     Fixed a bug when adding private key specs to an already working
     MDomain, see <https://github.com/icing/mod_md/issues/260>.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1893399 13f79535-47bb-0310-9956-ffa450edef68