ab: Allow for TLSv1.3 when the SSL library supports it.

ab: Allow for TLSv1.3 when the SSL library supports it.

When TLS1_3_VERSION is defined by the SSL library, bump the maximum TLS
protocol to that and use it for "-f ALL" or "-f TLSv1.3".

This mixes proposed patches from BZ 63594 and 64699.

BZ: 63594, 64699

Merge r1900157 from trunk:

Submitted by: abhilash <abhilash1232 gmail.com>
Submitted by: xiaolongx.jiang intel.com
Submitted by: ylavic
Reviewed by: ylavic, rpluem, jorton

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1901203 13f79535-47bb-0310-9956-ffa450edef68