crypto/x509: ignore non-critical email constraints
crypto/x509: ignore non-critical email constraints

Previously we tried to parse email constraints as the maximum base
distance (which is unused and must be omitted according to RFC 5280)
because they share the same tag number. Rather than complicate the
code further, this CL just ignores the unused values.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/6552044

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