tests: kill_functions.sh needs gawk (skip if sourced)

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>
tests: fix kill problems

There were several failures (missing tmp files, missing pids) on travis
build farm which I couldn't track down completely. Here we fix some
possible issues:
 - mktemp -u is unsafe, maybe even in practice if there is a bad random
   generator
 - make sure that mktemp does not give us pure integer filnames which
   would not work for "kill by-name"
 - avoid using trap, could be that it removed tmp files on wrong signals

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>