utrace: Forward SIGTERM to the traced process

When a service is started with "/etc/init.d/<service> trace" or when
it has seccomp enabled (i.e. runs under seccomp-trace), stopping the
service with "/etc/init.d/<service> stop" stops only the tracer. The
service itself continue executing. This patch ensures that the service
is terminated as well.

Signed-off-by: Michal Sojka <sojkam1@fel.cvut.cz>