Part of #2080.
This change vendors `crypto/x509`, `crypto/x509/pkix`, and `encoding/asn1` from 1d5f6a765d. That commit is a direct child of the Go 1.5.4 release tag, so it contains the same code as the current Go version we are using. In that commit I rewrote imports in those packages so they depend on each other internally rather than calling out to the standard library, which would cause type disagreements.
I changed the imports in each place where we're parsing CSRs, and imported under a different name `oldx509`, both to avoid collisions and make it clear what's going on. Places that only use `x509` to parse certificates are not changed, and will use the current standard library.
This will unblock us from moving to Go 1.6, and subsequently Go 1.7.
Roland Bracewell Shoemaker