RFR: JDK-8272984: javadoc support for SOURCE_DATE_EPOCH [v2]

[Jonathan Gibbons]

> Please review a relatively simple patch to support the standard SOURCE_DATE_EPOCH environment variable, used by those who are interested in reproducible builds. (See https://reproducible-builds.org)

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

  ignore invalid settings

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6905/files
  - new: https://git.openjdk.java.net/jdk/pull/6905/files/385f785f..327ee69b

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6905&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6905&range=00-01

  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6905.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6905/head:pull/6905

PR: https://git.openjdk.java.net/jdk/pull/6905