RFR: 8142979: Add programmatic enable/disable methods to Unified Logging

[Marcus Larsson]

Hi,

Please review the following patch adding support for configuration of 
the stdout unified logging output from within the VM itself.

The patch adds the configure_stdout() function, allowing a set of tags 
to be set to the given level on stdout.
To disable logging the level is simply specified as LogLevel::Off.

The patch depends on the fix for 8142952 [0] that's currently out on 
review, so the configuration is correctly aggregated rather than reset.

Webrev:
http://cr.openjdk.java.net/~mlarsson/8142979/webrev.00/

Issue:
https://bugs.openjdk.java.net/browse/JDK-8142979

Thanks,
Marcus

[0]: https://bugs.openjdk.java.net/browse/JDK-8142952