Allow specifying CRL directory

Allow specifying CRL directory

Add another method to specify CRLs, hashed directory method, for both
server and client side.  This offers a means for server or libpq to
load only CRLs that are required to verify a certificate.  The CRL
directory is specifed by separate GUC variables or connection options
ssl_crl_dir and sslcrldir, alongside the existing ssl_crl_file and
sslcrl, so both methods can be used at the same time.

Author: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/20200731.173911.904649928639357911.horikyota.ntt@gmail.com