The format for a CSR is horribly underspecified and we had a mistake.
The code was parsing the attributes from the CSR as a
pkix.AttributeTypeAndValueSET, which is only almost correct: it works so
long as the requested extensions don't contain the optional “critical”
flag.

Unfortunately this mistake is exported somewhat in the API and the
Attributes field of a CSR actually has the wrong type. I've moved this
field to the bottom of the structure and updated the comment to reflect
this.

The Extensions and other fields of the CSR structure can be saved
however and this change does that.

Fixes #11897.

Change-Id: If8e2f5c21934800b72b041e38691efc3e897ecf1
Reviewed-on: https://go-review.googlesource.com/12717Reviewed-by: Rob Pike <r@golang.org>