crl-checker: allow disabling signature validation (#6432)
crl-checker: allow disabling signature validation (#6432)

Allow the -issuer flag to take the special value "-", which causes no
issuer file to be loaded and no CRL signatures to be validated. Also
improve the error message if no -issuer flag is provided at all.

Fixes #6409

letsencrypt/boulder,letsencrypt/boulder,letsencrypt/boulder