RFR: JDK-8272984: javadoc support for reproducible builds [v3]

[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.

Jonathan Gibbons has updated the pull request incrementally with one additional commit since the last revision:

  Fix typo
  Improve command-line help message
  Clean up imports in new test

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7171/files
  - new: https://git.openjdk.java.net/jdk/pull/7171/files/65ab48ed..037cabbf

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7171&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7171&range=01-02

  Stats: 5 lines in 3 files changed: 0 ins; 3 del; 2 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