OCSP stapling: create GnuTLSUseStapling config directive

Signed-off-by: Michael Tänzer <neo@nhng.de>
add: Initial hook for OCSP stapling
add: Additional fields for tracking OCSP response information
add: Initial callbacks to return OCSP response
add: Include cURL to be linked in
add: Basic functionality to do OCSP requests and read expiry information back

This source is based on the OCSP client sample in GnuTLS 3.2.x
add: Actually reply to OCSP stapling requests with the cached data and refresh the response if necessary
add: Actually check the response belongs to our request
Make Apache 2.4 display the correct module in error logs including the new OCSP stuff