Merge of r4813, r4814, r4818, r4819, r4820, r4823: mail fixes.

Merge of r4813, r4814, r4818, r4819, r4820, r4823: mail fixes.

*) Corrected the directive name in the ngx_mail_auth_http_module
   error message.

*) Don't let the well-known port in the "listen" directive to
   override the already set "protocol".

*) Fixed sorting of listen addresses (ticket #187).  For http module
   this problem was already fixed in r4756.

*) Removed a stale "AF_INET only" comment.  IPv6 client connections
   in mail modules have been supported since r2856.

*) Fixed handling of AF_UNIX addresses in "listen".  This makes AF_UNIX
   addresses in mail officially supported.

*) Whitespace fix.