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>