RFR: JDK-8288003: log events for os::dll_unload [v4]

[Matthias Baesken]

> Currently we only log events for os::dll_load, but not for os::dll_unload, this patch adds it. On some platforms (Linux/Windows) we can use OS APIs (e.g. dlinfo on Linux) to log the path of the unloaded shared lib.

Matthias Baesken has updated the pull request incrementally with one additional commit since the last revision:

  dll_path on Linux - add NULL check, change special output to not available

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Changes:
  - all: https://git.openjdk.org/jdk/pull/9101/files
  - new: https://git.openjdk.org/jdk/pull/9101/files/dbdca1ee..fa7b3b7b

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=9101&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=9101&range=02-03

  Stats: 6 lines in 1 file changed: 2 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/9101.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9101/head:pull/9101

PR: https://git.openjdk.org/jdk/pull/9101