This commit implements a test for the signal feature of initctl. The
existing service used in tests (`common/service.sh`) has been extended
with a signal trap handler. When SIGUSR1 is received, it will write the
string `'USR1'` to `/tmp/usr1.log`. The test will run `initctl signal
service.sh SIGUSR1` and assert that the contents of the file
`/tmp/usr1.log` really is `USR1`.
