signatureAlgorithmDetails already knows the hash function for each
signature algorithm so there's no point in duplicating that. Also, check
that the public key type of the signature algorithm actually matches the
given public key.

Change-Id: I7aab4ea71691fb815d67ba790b721ce02de11b85
Reviewed-on: https://go-review.googlesource.com/57211Reviewed-by: Adam Langley <agl@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>