libjsig should ignore non-modifying sigaction calls

Found during code review of JDK-8292695.

We have two bugs in libjsig when we install hotspot signal handlers. Relevant code in libjsig:

```
int sigaction(int sig, const struct sigaction *act, struct sigaction *oact) {
<snip>

  sigused = sigismember(&jvmsigs, sig);
  if (jvm_signal_installed && sigused) {
    /* jvm has installed its signal handler for this signal. */
    /* Save the handler. Don't really install it. */
    if (oact != NULL) {
      *oact = sact[sig];
    }
    if (act != NULL) {
      sact[sig] = *act;
    }

    signal_unlock();
    return 0;
  } else if (jvm_signal_installing) {
    /* jvm is installing its signal handlers. Install the new
     * handlers and save the old ones. */
    res = call_os_sigaction(sig, act, &oldAct);
    sact[sig] = oldAct;
    if (oact != NULL) {
      *oact = oldAct;
    }

    /* Record the signals used by jvm. */
    sigaddset(&jvmsigs, sig);

    signal_unlock();
    return res;
  }
<snip>
}
```

Bug 1: we change state even if the sigaction call failed
Bug 2: we change state even if the sigaction call was a non-modifying one (act == NULL)

The latter is usually no problem since hotspot always calls `sigaction()` in pairs when installing a signal: first with NULL to get the old handler, then with the real handler. But this is not always true. If `AllowUserSignalHandlers` is set, and we find a custom handler is present, we will not override it:

```
void set_signal_handler(int sig, bool do_check = true) {
  // Check for overwrite.
  struct sigaction oldAct;
  sigaction(sig, (struct sigaction*)NULL, &oldAct); <<<<< first sigaction call, libjsig now remembers signal as set

  // Query the current signal handler. Needs to be a separate operation
  // from installing a new handler since we need to honor AllowUserSignalHandlers.
  void* oldhand = get_signal_handler(&oldAct);
  if (!HANDLER_IS_IGN_OR_DFL(oldhand) &&
      !HANDLER_IS(oldhand, javaSignalHandler)) {
    if (AllowUserSignalHandlers) {
      // Do not overwrite; user takes responsibility to forward to us.
      return;
```

That means:
- we still have the original custom handler in place
- but we already called sigaction, albeit with NULL, but libjsig now assumes that hotspot installed a handler itself.

The result is that any further attempts to change the signal handler, whether by hotspot or by user code, will be prevented by libjsig. Any further non-modifying sigaction calls will return the original - still installed - custom handler.

Admittedly, the error is very exotic. Users would have to set AllowUserSignalHandlers and preload libjsig, and *then* attempt to modify signal handlers after JVM initialization. But it is confusing, and a potential source for other errors. In hotspot, nobody counts on a non-modifying sigaction query changing program state somewhere.

This seems to be an old bug, I see it in at least JDK 8. Did not look further into the past than that.