Rework the timeout of the CGI script.

The previous implementation was fine to catch timeout of third-party
tools (like dot), but to good to catch timeout in Spot itself, because
Python will not deliver a SIGALRM while a native function (e.g. Spot's
translation) is running.  So we fork() the Python process, with a
parent that does nothing but wait for the termination of the child or
for an alarm.  On SIGALRM, the parent kills all children.

* wrap/python/ajax/spot.in: Adjust to fork.
* wrap/python/tests/alarm.py: New test file to test this
scenario in a more controled environment.
* wrap/python/tests/Makefile.am: Add it.

wrap/python/tests/Makefile.am

@@ -30,6 +30,7 @@ TESTS_ENVIRONMENT = ./run
 check_SCRIPTS = run
 
 TESTS = \
+  alarm.py \
   ltlsimple.py \
   ltlparse.py \
   bddnqueen.py \