RFR: 8146948: Add help information to log tags

[Marcus Larsson]

Hi,

Please review the following patch to add descriptions to logging tagsets.

Summary:
Tagsets may now be given a description in 
logTagSetDescriptions.inline.hpp. All described tagsets are listed in 
the -Xlog:help output as well as in the LogConfiguration::describe 
output. The patch also adds trace level logging that lists *all* tagsets 
in the VM (-Xlog:logging=trace). The previous 'logging=trace logging' 
has been moved to 'logging=debug'.

Example log output:

...
[0.544s][debug][logging] Described tag combinations:
[0.544s][debug][logging]  logging: Logging for the log framework itself
...
[0.544s][trace][logging] All available tag sets: arguments, biasedlocking, classinit, classload, classload+constraints, classload+preorder, classloaderdata, classpath, classresolve, classunload, constraints, defaultmethods, ergo, exceptions, gc, gc+age, gc+alloc, gc+alloc+region, gc+barrier, gc+bot, gc+classhisto, gc+classhisto+start, gc+compaction, gc+cpu, gc+ergo, gc+ergo+cset, gc+ergo+heap, gc+ergo+ihop, gc+ergo+refine, gc+freelist, gc+freelist+census, gc+freelist+stats, gc+heap, gc+heap+coops, gc+heap+exit, gc+heap+region, gc+humongous, gc+ihop, gc+jni, gc+liveness, gc+marking, gc+marking+start, gc+metaspace, gc+metaspace+alloc, gc+metaspace+freelist, gc+phases, gc+phases+start, gc+phases+task, gc+phases+verify, gc+phases+verify+start, gc+plab, gc+promotion, gc+ref, gc+ref+start, gc+refine, gc+region, gc+remset, gc+remset+exit, gc+remset+scrub, gc+scavenge, gc+start, gc+state, gc+stats, gc+stringdedup, gc+stringtable, gc+survivor, gc+sweep, gc+task, gc+task+stats, gc+task+thread, gc+task+time, gc+tlab, gc+verify, gc+verify+start, gc+workgang, heap+ergo, itables, jfr, jfr+instrumentation, jfr+types, logging, logging+test, modules, monitorinflation, monitormismatch, os, os+cpu, os+thread, pagesize, protectiondomain, ref, safepoint, safepointcleanup, stacktrace, startuptime, verification, vmoperation, vtables


Webrev:
http://cr.openjdk.java.net/~mlarsson/8146948/webrev.00/

Issue:
https://bugs.openjdk.java.net/browse/JDK-8146948

Testing:
Internal VM tests through RBT

Thanks,
Marcus