crypto/x509: re-unify the OS X code paths for loading roots

[bradfitz]

This bug is about fixing #16473 properly for Go 1.8.

OS X 10.8 crashes for some reason when loading cert roots using code which works fine from OS X 10.9+.

https://golang.org/cl/25233 + https://golang.org/cl/25281 went in as conservative fixes for Go 1.7, but @hinman has a better patch (https://golang.org/cl/25190) which needed more review and somebody understanding what was happening on OS X 10.8. (see confusion in #16473 (comment) for instance)

[rsc]

Let's just wait until we can retire OS X 10.8.

[ianlancetaylor]

Sounds like there is nothing to do here.