Add --[enable|disable]-v4-mapped configure option to control

Add --[enable|disable]-v4-mapped configure option to control
whether or not Apache expects to handle IPv4 connections
on IPv6 listening sockets.  Either setting will work on
systems with the IPV6_V6ONLY socket option.  --enable-v4-mapped
must be used on systems that always allow IPv4 connections on
IPv6 listening sockets.

Note: As the ssl config file is not automatically generated and
      it is expected to require editing anyway to work, the only
      change there was to suggest the required Listen statements
      in a comment.

PR:   PR 14037 (Bugzilla), PR 7492 (Gnats), various dups of these PRs

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@97516 13f79535-47bb-0310-9956-ffa450edef68