crypto/x509: be strict about trailing data.

The X.509 parser was allowing trailing data after a number of structures
in certificates and public keys. There's no obvious security issue here,
esp in certificates which are signed anyway, but this change makes
trailing data an error just in case.

Fixes #10583

Change-Id: Idc289914899600697fc6d30482227ff4bf479241
Reviewed-on: https://go-review.googlesource.com/9473Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>