config: parallel-safe param.h makefile rule

Generate the param.h.new temporary file in a parallel-safe
way.  The rule to generate the three copies of param.h can
run at the the same time under a parallel make, clobbering
the param.h.new temporary file. Instead of creating this file
inline, create a common rule to generate the temporary file
once.

Change-Id: I823b6a55f3168e991b64660bfe51303d43f693a9
Reviewed-on: http://gerrit.openafs.org/10516
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>