Integrate reported quality issues

[matthiasblaesing]

Closes: #1560
Closes: #1561

[matthiasblaesing]

@dbwiddis could you please have a look at this? There are differences regarding the suggested approach by @sebbASF, but the core matches. Where I took a different route:

• NativeLibraryDisposer.handle does not need to be volatile. All access to that variable is guarded by the synchronized keyword. This establishes a happens-before relationship for all other synchronized blocks on the same monitor.
• NativeLibrary#callFlags and NativeLibrary#options are now private. I did not find a use-site for callFlags outside NativeLibrary and NativeLibrary#options has a public accessor getOptions, which should be used

[sebbASF]

Good point; not sure how I missed that. I no longer think that handle needs to be volatile.

[dbwiddis]

Looks sane to me. One suggestion (see also #1571)

[dbwiddis]

This map is still mutable; if someone modified the library map (e.g. getOptions.clear() that would echo here.

Now that we are on JDK8 I'd suggest making this version of it an unmodifiable copy. (I'd suggest making the getter return unmodifiable but that's probably a breaking change.)

Suggested change

	this.options = library.getOptions();
	this.options = Map.copyOf(library.getOptions());

[matthiasblaesing]

Thanks for the review. I'll integrate as if for now, I think the discussion about potentially user facing changes needs some more time.