crypto/x509: limit number of signature checks for each verification
That number grows quadratically with the number of intermediate
certificates in certain pathological cases (for example if they all have
the same Subject) leading to a CPU DoS. Set a fixed budget that should
fit all real world chains, given we only look at intermediates provided
by the peer.
The algorithm can be improved, but that's left for follow-up CLs:
* the cache logic should be reviewed for correctness, as it seems to
override the entire chain with the cached one
* the equality check should compare Subject and public key, not the
whole certificate
* certificates with the right SKID but the wrong Subject should not
be considered, and in particular should not take priority over
certificates with the right Subject