`net stop` return code is misleading, check stdout

When `net stop` gives up waiting for a service to stop, it still has
retcode 0, so we need to check the actual output to tell if it worked or
not. If we see "service was stopped" we consider it good, otherwise
start polling.

Sample `cmd.run_all` output for the `net stop` failure case:

    pid:
        6360
    retcode:
        0
    stderr:

    stdout:
        The NSClient++ (Win32) service is stopping........
        The NSClient++ (Win32) service could not be stopped.

Sample `cmd.run` output for the when `net stop` works as intended:

    The NSClient++ (Win32) service is stopping...
    The NSClient++ (Win32) service was stopped successfully.