crypto/x509: check the Key Usage extension

[FiloSottile]

We currently ignore the Key Usage extension because some old roots had gotten it wrong. Things probably improved by now and we can take a stab at following the spec again. AFAIK other verifiers do check them.

[gopherbot]

Change https://golang.org/cl/246877 mentions this issue: crypto: check the Key Usage extension

[SparrowLii]

I searched the crypto pkg and found there are two functions in crypto which relate to KU extension checking:
CheckSignature(x509.go 823) and verifyHandshakeSignature(auth.go 22). But it seems hard to check the KU inside the func body. I think maybe we can check KU before use one PublicKey.