Go 1.10 requires that SANs in certificates are valid. However, a
non-trivial number of (generally non-WebPKI) certificates have invalid
strings in dnsName fields and some have even put those dnsName SANs in
CA certificates.

This change defers validity checking until name constraints are checked.

Fixes #23995, #23711.

Change-Id: I2e0ebb0898c047874a3547226b71e3029333b7f1
Reviewed-on: https://go-review.googlesource.com/96378
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>