fix: assign success status when displaying help usage via -h or -help

[KristijanZic]

Added success status assignment before exiting help usage.

In environments with set -e -o pipefail returning a non-zero exit code like -1 or (255 when -1 gets wrapped around to the maximum unsigned 8-bit value) results in help tests invocations failing like in this one:

set -e -o pipefail
filemonitor -h | grep -F "FileMonitor"

the issue is, some environments like Nix test have set -e -o pipefail set by default like here:

    tests = {
      help = testers.runCommand {
        name = "filemonitor-help-test";
        buildInputs = [ finalAttrs.finalPackage ];
        script = ''
          filemonitor -h | grep -F "FileMonitor (v${finalAttrs.version})"
          touch $out
        '';
      };
    };

And while one can work around this issue, a proper fix would be much better.