mod_ssl: Check the SSLProtocol directive when loading the configuration

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