Strict DER signatures.

Requires every signature to follow one strict, standard format, so all nodes agree on which signatures are valid.

Nodes relied on an outside code library, OpenSSL, to read signatures, and it accepted slightly malformed ones. When an update to it changed this, some nodes rejected the chain. This rule accepts only signatures in the exact standard format (DER), so validity no longer depends on that library. Once the whole network enforced it, its author disclosed that it had also, indirectly, fixed a flaw he had found earlier in the rules every node must agree on.