[jdk11u-dev] RFR: 8240256: Better resource cleaning for SunPKCS11 Provider

[Martin Doerr]

Backport of JDK-8240256. Actual fix applies cleanly, but tests need resolution:

- PKCS11Test.java: Needed to integrate manually. Can't remove imports because they are still needed in 11u.
- MultipleLogins.java: `!Reference.refersTo(null)` is not available in 11. Replaced by `Reference.get() != null`
- ForceGC.java is needed. Copied from jdk/head.

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Commit messages:
 - Backport bdeaeb47d0155b9f233274cff90334e8dd761aae

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/352/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=352&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8240256
  Stats: 650 lines in 12 files changed: 546 ins; 66 del; 38 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/352.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/352/head:pull/352

PR: https://git.openjdk.java.net/jdk11u-dev/pull/352