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
Remove old-fashioned identifiers for the cache mechanism we no longer need
Include a central function to get the cache identifier for OCSP responses bases on the certificate being checked
Introduce minimalistic wrappers to the OCSP caching interface
Change existing OCSP code to use the new socache-based interface to store responses
Make sure the Cache instance is mirrored over to each server that's trying to access it
Port change of module tag to stapling branch
Be more verbose with OCSP cache requests