Derive application name from sys.argv[0].
Not all systems have SIGUSR1

I worked around that in the actual code, but neglected to address it
for the unit tests.  This works around the fact for the unit tests.
It also removes test_init_failusr1() because that can't be properly
tested; the corresponding code is covered with "no cover" pragmas.
Guards around SIGUSR1 are uncoverable WRT tests.
Fix an error in start()

The NotifierServer.start() method tried to use "manager.connect()" to
start the manager, when "self._manager.connect()" is correct.
Correct a left-over mention of _running.
No test coverage for notification_server().
Add tests for NotifierServer.