crypto/x509: don't apply name constraints to non-terminal self-issued certs

[cpu]

RFC 5280 §4.2.1.10 when spec'ing Name Constraints for the 5280 profile says (emphasis mine):

Name constraints are not applied to self-issued certificates (unless
the certificate is the final certificate in the path).

This is tested by the x509-limbo rfc5280::nc::permitted-self-issued testcase.

Go crypto/x509 should not apply name constraints to self-issued certs in the path unless they're the terminal certificate.

Relates to #65085 & https://go-review.googlesource.com/c/go/+/783980

[gabyhelp]

Related Issues

• crypto/x509: reject end-entity certs w/ name constraint ext. #79742
• crypto/x509: don't count self-signed certs in pathlen budget #79745
• crypto/x509: Certificate.Verify does not enforce name constraints correctly #59171 (closed)
• crypto/x509: Verify should error if non-CA certificate contains name constraints #71795
• crypto/x509: reject non-profile minimum/maximum in GeneralSubtree name constraints #77826
• crypto/x509: enforce Name Constraints on intermediates with SANs #45856 (closed)
• crypto/x509: reject CA w/ empty subject #79744

Related Code Changes

• crypto/x509: Adding Name Constraints
• crypto/x509: enforce all name constraints and support IP, email and URI constraints
• crypto/x509: properly apply name constrains to roots and intermediates

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[mrkfrmn]

cc @golang/security