Currently avocado adds custom handlers of SIGINT and SIGUSR1 in order to
produce traceback and notify about user interaction or timeout. This
could be missleading in case the test uses those signals and potentially
dangerous as some tests assume default behavior.

This patch removes the custom handling of SIGINT and SIGUSR1 and
reports the failure in `job.log`. Additionally it tries to inject the
error message in the test output, if status.logfile available.

In order to keep the useful traceback in case of interruption, this
patch overrides the default SIGTERM handler. The default behavior of
SIGTERM is to die, our custom handler raises SystemExit with info saying
the test was interrupted by sigterm, which should generate traceback and
finish. The runner then changes the result to INTERRUPTED, so even
when the test modifies the SIGTERM handler, we get the correct status.

Worth mentioning that in case test ignores SIGTERM, SIGKILL is emitted
by the runner, so this should be safe approach.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>