RFR: 8274793: Suppress warnings on non-serializable non-transient instance fields in sun.net

[Joe Darcy]

Augmentations to javac's Xlint:serial checking are out for review (#5709) and the sun.net classes in this PR would need some changes to pass under the expanded checks.

The changes are to suppress warnings where non-transient fields in serializable types are not declared with a type statically known to be serializable. That isn't necessarily a correctness issues, but it does merit further scrutiny.

The changes in the PR are consistent with similar changes made in JDK libraries under the umbrella issue JDK-8231641.

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Commit messages:
 - 8274793: Suppress warnings on non-serializable non-transient instance fields in sun.net

Changes: https://git.openjdk.java.net/jdk/pull/5826/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5826&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274793
  Stats: 3 lines in 2 files changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5826.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5826/head:pull/5826

PR: https://git.openjdk.java.net/jdk/pull/5826