crypto/x509: enforce all name constraints and support IP, email and U…

…RI constraints

This change makes crypto/x509 enforce name constraints for all names in
a leaf certificate, not just the name being validated. Thus, after this
change, if a certificate validates then all the names in it can be
trusted – one doesn't have a validate again for each interesting name.

Making extended key usage work in this fashion still remains to be done.

Updates golang#15196

Change-Id: I72ed5ff2f7284082d5bf3e1e86faf76cef62f9b5
Reviewed-on: https://go-review.googlesource.com/62693
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>