sslutil: allow multiple fingerprints per host Certificate pinning via [hostfingerprints] is a useful security feature. Currently, we only support one fingerprint per hostname. This is simple but it fails in the real world: * Switching certificates breaks clients until they change the pinned certificate fingerprint. This incurs client downtime and can require massive amounts of coordination to perform certificate changes. * Some servers operate with multiple certificates on the same hostname. This patch adds support for defining multiple certificate fingerprints per host. This overcomes the deficiencies listed above. I anticipate the primary use case of this feature will be to define both the old and new certificate so a certificate transition can occur with minimal interruption, so this scenario has been called out in the help documentation.