RFR: 8274227: Remove "impl.prefix" jdk system property usage from InetAddress

[Aleksei Efimov]

The following fix proposes to remove usages of `"impl.prefix"` JDK system property from the `java.net.InetAddress` class.
This system property  is used to locate concrete implementations of the package private "java.net.InetAddressImpl" interface.

The list of changes:
 -  `impl.prefix` usages are removed
 -  `InetAddressImpl` made sealed interface by only allowing default implementations available in 'java.net' package: `Inet4AddressImpl` and `Inet6AddressImpl`.

tier1-tier3 tests show no failures  with this fix.

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Commit messages:
 - Merge branch 'master' into JDK-8274227_remove_impl_prefix
 - 8274227: Seal java.net.InetAddressImpl
 - 8274227: Remove "impl.prefix" jdk system property usage from InetAddress

Changes: https://git.openjdk.java.net/jdk/pull/5755/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5755&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274227
  Stats: 53 lines in 4 files changed: 0 ins; 45 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5755.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5755/head:pull/5755

PR: https://git.openjdk.java.net/jdk/pull/5755