upstream: Client-side workaround for a bug in OpenSSH 7.4: this release

allows RSA/SHA2 signatures for public key authentication but fails to
advertise this correctly via SSH2_MSG_EXT_INFO. This causes clients of these
server to incorrectly match PubkeyAcceptedAlgorithms and potentially refuse
to offer valid keys.

Reported by and based on patch from Gordon Messmer via bz3213, thanks
also for additional analysis by Jakub Jelen. ok dtucker

OpenBSD-Commit-ID: d6d0b7351d5d44c45f3daaa26efac65847a564f7
