Summary
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New system properties available to configure Unicode Locale Extensions

Problem
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In order for users to specify the default Unicode Locale Extensions through the command line option, new system properties which are analogous to "user.language" should be defined.

Solution
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Introducing the following new system properties:

- user.extensions
- user.extensions.display
- user.extensions.format

The value for these properties should take the form of BCP47's "extension" (https://tools.ietf.org/html/bcp47): 
 
> extension = singleton 1*("-" (2*8alphanum))

The properties will be read at the JVM startup time (Locale class' static initializer) as well as other Locale related system properties (user.language/region/variant). Later alterations to the properties will have no effect. If the specified extension is invalid, it will be ignored.

Specification
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Since this enhancement is adding jdk system properties, no specification change is needed.