A CSR containing challengePassword or unstructuredName Attributes
(included in default OpenSSL prompts) would break ASN.1 parsing.
This updates the parsing structures to allow but then ignore these
fields.

See this CFSSL issue: https://github.com/cloudflare/cfssl/issues/115

Change-Id: I26a3bf1794589d27e6e763da88ae32276f0170c7
Reviewed-on: https://go-review.googlesource.com/8160Reviewed-by: Adam Langley <agl@golang.org>