RFR: 8260693: Provide the support for specifying a signer in keytool -genkeypair [v4]

[Hai-May Chao]

> Please review the changes that adds the -signer option to keytool -genkeypair command. As key agreement algorithms do not have a signing algorithm, the specified signer's private key will be used to sign and generate a key agreement certificate.
> CSR review is at: https://bugs.openjdk.java.net/browse/JDK-8264325

Hai-May Chao has updated the pull request incrementally with one additional commit since the last revision:

  update with review comments

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3281/files
  - new: https://git.openjdk.java.net/jdk/pull/3281/files/d63818ba..157601f2

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3281&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3281&range=02-03

  Stats: 39 lines in 4 files changed: 7 ins; 19 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3281.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3281/head:pull/3281

PR: https://git.openjdk.java.net/jdk/pull/3281