use ECDSA, e.g., if it's all we have

[richsalz]

From @vdukhovni in the thread on #1597

we must not fail to use an ECDSA certificate (supposing it is the only one we have) even if we share no curves with the client. That's a more complex fix, since it will likely require a second pass over the shared ciphersuites with a less stringent filter. I'd suggest a separate PR for that. This is also needed for supported signature algorithms (SHA1 cert when client only offers SHA256, ...)

[richsalz]

We will not delay 1.1.1 for this.

[vdukhovni]

A simple short-term fix is possible. If we have exactly one certificate, then don't check its conformance with client's "supported " extensions. Provided the leaf public key is acceptable, proceed with the handshake. This will cover the vast majority of cases. In most of the other cases we'll have a vanilla RSA cert and an ECDSA cert, and the vanilla RSA cert will be fine.