Build and use roken's mkstemp

Build and use roken's mkstemp

Add roken's mkstemp to the build on all platforms, either through
configure (Unix) or explicitly (Windows). Change all callers so
that mkstemp is used unconditionally, rather than falling back
to more risky alternatives.

This is based on original change from Russ Allbery submitted as
http://gerrit.openafs.org/2146

Change-Id: Ia1def41e438e6517b3e7cd5790df3c99eddb8ef6
Reviewed-on: http://gerrit.openafs.org/3398
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>