RFR: 8201450: Provide access to LogHandle tagset

[Kim Barrett]

Please review this small addition to the LogHandle API, providing
access to its _tagset member. This supports constructing a
LogStream from a log level and a LogHandle, among other things.

CR:
https://bugs.openjdk.java.net/browse/JDK-8201450

Webrev:
http://cr.openjdk.java.net/~kbarrett/8201450/open.00/

Testing:
Built on on Oracle supported platforms, hs-tier1.