RFR: 8283772: Make sun.net.dns.ResolverConfiguration sealed

[Aleksei Efimov]

The following fix seals `sun.net.dns.ResolverConfiguration` abstract class.
`sun.net.dns.ResolverConfigurationImpl` is the only permitted subclass which has two O/S specific implementations: for `Windows` and `Unix` architectures. Both of them are marked as `final`.

Testing: `jdk-tier1`, `jdk-tier2`, `jdk-tier3` and `java.naming` JCK tests show no failures with the change.

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Commit messages:
 - 8283772: Make sun.net.dns.ResolverConfiguration sealed

Changes: https://git.openjdk.java.net/jdk/pull/8006/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8006&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283772
  Stats: 6 lines in 3 files changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8006.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8006/head:pull/8006

PR: https://git.openjdk.java.net/jdk/pull/8006