mod_ssl: Check the SSLProtocol directive when loading the configuration

Previously, the SSLProtocol directive was checked at runtime. Apache quit if
the directive contained an invalid combination of protocols, and logged the
message "AH02231: No SSL protocols available [hint: SSLProtocol]".

With this change, most invalid SSLProtocol directives are detected when
checking the configuration, e.g. with \"httpd -t -f httpd.conf\".

Examples of invalid protocol combinations that are caught:
* SSLProtocol "-TLSv1"
* SSLProtocol "-all"
* SSLProtocol "TLSv1.2 -TLSv1.2"

Submitted by: Michael Kaufmann <mail michael-kaufmann.ch>
Github: closes #523


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1924955 13f79535-47bb-0310-9956-ffa450edef68

Author: notroj

changes-entries/ssl-check-protoconf.txt

@@ -0,0 +1,3 @@
+  *) mod_ssl: Fail when parsing SSLProtocol if the configuration 
+     would prevent use of all protocols.
+     [Michael Kaufmann <mail michael-kaufmann.ch>]

modules/ssl/ssl_engine_config.c

@@ -1610,6 +1610,11 @@ static const char *ssl_cmd_protocol_parse(cmd_parms *parms,
         }
     }
 
+    if (*options == SSL_PROTOCOL_NONE) {
+        return "SSLProtocol: No SSL protocols available";
+    }
+
+
     return NULL;
 }