nginx_voms: Enable CRL support in the SSL context used for TLS handshake
Currently, CRLs are honoured in VOMS AC validation, but ignored for TLS handshake (i.e., by the core NGINX SSL module configuration).
CRLs must be honoured also at TLS handshake time. The easiest way to do this is probably leveraging the ssl crl directive, which however requires a single file containing all CRLs (while typically in the trust anchors structure we use CRLs are placed in separate files).