RFR: 8142952: Unified Logging framework does not allow multiple -Xlog: arguments

[Marcus Larsson]

Hi,

Please review the following patch to enable multiple -Xlog arguments to 
aggregate the log configuration.

Summary:
Tag sets that are unaffected by a -Xlog argument will now be left 
untouched, enabling them to keep settings from a previous -Xlog argument.

To be able to print the current log configuration, the _config_string on 
LogOutputs has to be derived from each enabled tag set and its level, 
instead of the previous copying of the -Xlog argument. This means that 
-Xlog:all:stdout sets the _config_string for stdout to the explicit list 
of all tag sets in the VM, with level set to info.

Webrev:
http://cr.openjdk.java.net/~mlarsson/8142952/webrev.00/

Issue:
https://bugs.openjdk.java.net/browse/JDK-8142952

Testing:
Included test through JPRT.

Thanks,
Marcus