Also, have valid_chain_p return boolean, add some debug logging and
detect invalid signature types instead of crashing.

This filters out certificates with signing algorithms that we can't
handle.

Also, make unit tests better.

Trying to figure out why public_key:verify isn't found in docker images.

Even canoncalized versions of this data mismatch in otherwise proper
chains. Since we're not here to validate chains for any other reasons
than attribution and spam control, let's stop validate
cert.issuer==candidate.subject. We still verify the cryptographic
chain with signatures of tbsCertificates of course.

Resolves CATLFISH-19.

Makefile target 'check' runs them.

OTP cert validation is too strict. Let's see if this is forgiving
enough for our needs.

Also, move all cert reading from disk to x509.erl.

Closes CATLFISH-5.