Step 2 of stream-lining parameter parsing

- do parameter validity checks in parser
- two passes instead of multiple (and theoretically quadratic) passes
  when checking parameters
- removes the need for OKEY and some ONONAME nodes in those passes

This removes allocation of ~123K OKEY (incl. some ONONAME) nodes
out of a total of ~10M allocated nodes when running make.bash, or
a reduction of the number of alloacted nodes by ~1.2%.

Change-Id: I4a8ec578d0ee2a7b99892ac6b92e56f8e0415f03
Reviewed-on: https://go-review.googlesource.com/20748Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Robert Griesemer <gri@golang.org>