RFR: JDK-8272984: javadoc support for reproducible builds

[Jonathan Gibbons]

Please review a javadoc update to support a new `--date` option to support reproducible builds.

This pull request supersedes https://github.com/openjdk/jdk/pull/6905. In that PR, the `SOURCE_DATE_EPOCH` environment variable was used to provide the time stamp, but review feedback suggested the use of a new command-line option, `--date`.  The format of the argument of the `--date` option is that same as that of similar options for the _jar_ and _jmod_ tools, and the code to handle the value is based on code in the _jar_ tool.

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Commit messages:
 - JDK-8272984: javadoc support for reproducible builds

Changes: https://git.openjdk.java.net/jdk/pull/7171/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7171&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272984
  Stats: 243 lines in 9 files changed: 229 ins; 1 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7171.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7171/head:pull/7171

PR: https://git.openjdk.java.net/jdk/pull/7171