RFR: 8284105: Update security libraries to use sealed classes [v3]

[Sean Mullan]

> Please review these changes to update the security libraries to use sealed classes. See JEP 409 for more details on sealed classes.
> 
> No CSR is required as all the changes are to internal classes. A few classes that did not have subclasses were simply marked final instead of sealed.

Sean Mullan has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:

 - Make KrbAsRep final.
 - Merge
 - Make some classes package-private instead of sealed.
 - Update security libraries to use sealed classes.

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8165/files
  - new: https://git.openjdk.java.net/jdk/pull/8165/files/c27b1ba0..2a1ab2b8

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8165&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8165&range=01-02

  Stats: 4066 lines in 394 files changed: 1983 ins; 791 del; 1292 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8165.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8165/head:pull/8165

PR: https://git.openjdk.java.net/jdk/pull/8165