invoke setsid for each job

This should cause each jenkins job to reside in its own session. Linux'
automatic group scheduling should then cause jobs to be scheduled
equally rather than processes. The expected result is that highly
parallel jobs do not slow down single-cpu jobs.

bin/common-functions.sh

@@ -24,7 +24,7 @@ if [ "${0:0:5}" != "/tmp/" ] ; then
 	#  this hack makes it possible to overwrite long running scripts
 	#  anytime...)
 	echo "$(date) - start running \"$0\" as \"$TTT\" using \"$@\" as arguments."
-	$TTT "$@"
+	/srv/jenkins/bin/setsid.py $TTT "$@"
 	exit $?
 	# cleanup is done automatically via trap
 else

bin/setsid.py

+#!/usr/bin/python
+"""backport of util-linux' setsid -w to Debian stable"""
+
+import os
+import sys
+
+if __name__ == "__main__":
+    assert len(sys.argv) > 1
+    pid = os.fork()
+    if pid == 0:
+        os.setsid()
+        os.execvp(sys.argv[1], sys.argv[1:])
+    else:
+        cpid, status = os.wait()
+        assert cpid == pid
+        sys.exit(os.WEXITSTATUS(status))