OCSP stapling: ssl_stapling_file support.

Very basic version without any OCSP responder query code, assuming valid
DER-encoded OCSP response is present in a ssl_stapling_file configured.

Such file might be produced with openssl like this:

openssl ocsp -issuer root.crt -cert domain.crt -respout domain.staple \
             -url http://ocsp.example.com