RFR: JDK-8287735: Provide separate event category for dll operations

[Matthias Baesken]

Currently we have a number of event categories logged and displayed in the hs_err file like
Classes unloaded (0 events)
Classes redefined (0 events)
Internal exceptions (0 events)
VM Operations (0 events)
but the dll operations (like dll_load) go into the generic "Events" category where they usually are shifted out pretty fast.
Better provide a separate category DLL operations for them.
Currently the dll_load is logged.  Later dll_unload might be added.

-------------

Commit messages:
 - JDK-8287735

Changes: https://git.openjdk.java.net/jdk/pull/9058/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=9058&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8287735
  Stats: 27 lines in 6 files changed: 16 ins; 0 del; 11 mod
  Patch: https://git.openjdk.java.net/jdk/pull/9058.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/9058/head:pull/9058

PR: https://git.openjdk.java.net/jdk/pull/9058