Summary
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Control which implementation of `WatchService` is used on macOS.

Problem
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The existing `WatchService` implementation on macOS has its limitations, but it is quite stable and has characteristics users have become accustomed to over the years. The new FSEvents-based implementation, while providing much better performance, introduces a risk of new bugs and certainly changes the expected characteristics, which may not be desirable for every user.

Solution
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Instead of simply replacing the existing implementation with a new one, it is proposed to provide an opt-in new implementation with the help of a system property. This way, users not wishing to switch to the new implementation are not affected by the change and those willing to give the new implementation a try can do so with ease. Over time, the default may be re-considered in favor of the new FSEvents-based implementation.

Specification
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`FileSystem::newWatchService` returns an object of class `MacOSXWatchService` if `System.getProperty("sun.nio.fs.MacOSXWatchService.use")` returns non-`null` value and an object of class `PollingWatchService` otherwise (the status quo).