allow performing *only* fingerprint verification

Before if we verified a fingerprint we also required a valid certificate.
Valid in 'signed by a trusted CA, current time in valid timerange', not in the
sense of matching hostnames.
We should allow people to verify certificates via fingerprints that are totally
crap otherwise.
Merge pull request #444 from t-8ch/tls_fingerprints_noverify

Allow performing *only* fingerprint verification.