RFR: 8283060: RawNativeLibraries should allow multiple clients to load/unload the same library

[Mandy Chung]

A small improvement to `RawNativeLibraries`.   `RawNativeLibraries::load` returns the same `NativeLibrary` instance of a given path if it's called multiple times. This means that multiple clients have to coordinate that the last one using the loaded library is the one to close the library by calling `RawNativeLibraries::unload`; otherwise, an exception may be thrown.

This patch changes `RawNativeLibraries::load` to delegate to the underlying platform-specific library loading mechanism e.g. dlopen/dlclose that keeps the reference count.  So each call to `RawNativeLibraries::load` and `unload` is simply a call to dlopen and dlclose respectively.  Each client of calling `RawNativeLibraries::load` is responsible for calling `RawNativeLibraries::unload`.  This will be consistent with the current behavior when you call `load` and `unload` with a different `RawNativeLibraries` instance.

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Commit messages:
 - 8283060: RawNativeLibraries should allow multiple clients to load/unload the same library

Changes: https://git.openjdk.java.net/jdk/pull/8022/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8022&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283060
  Stats: 66 lines in 3 files changed: 43 ins; 10 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8022.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8022/head:pull/8022

PR: https://git.openjdk.java.net/jdk/pull/8022