RFR: 8173970: jar tool should have a way to extract to a directory [v4]

[Jaikiran Pai]

> Can I please get a review for this patch which proposes to implement the enhancement request noted in https://bugs.openjdk.java.net/browse/JDK-8173970?
> 
> The commit in this PR introduces the `-o` and `--output-dir` option to the `jar` command. The option takes a path to a destination directory as a value and extracts the contents of the jar into that directory. This is an optional option and the changes in the commit continue to maintain backward compatibility where the jar is extracted into current directory, if no `-o` or `--output-dir` option has been specified.
> 
> As far as I know, there hasn't been any discussion on what the name of this new option should be. I was initially thinking of using `-d` but that is currently used by the `jar` command for a different purpose. So I decided to use `-o` and `--output-dir`. This is of course open for change depending on any suggestions in this PR.
> 
> The commit in this PR also updates the `jar.properties` file which contains the English version of the jar command's `--help` output. However, no changes have been done to the internationalization files corresponding to this one (for example: `jar_de.properties`), because I don't know what process needs to be followed to have those files updated (if at all they need to be updated).
> 
> The commit also includes a jtreg testcase which verifies the usage of this new option.

Jaikiran Pai has updated the pull request incrementally with seven additional commits since the last revision:

 - Lance's review - include tests for --extract long form option
 - cleanup after each test
 - Adjust test for new error messages
 - Lance's review - add a code comment for xdestDir
 - Lance's review - updates to the help messages in jar.properties
 - Lance's review - add comment to the magic number 6 in the tests
 - Lance's review - add comments to test methods

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2752/files
  - new: https://git.openjdk.java.net/jdk/pull/2752/files/3df602d2..b5de6e3d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2752&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2752&range=02-03

  Stats: 139 lines in 3 files changed: 98 ins; 0 del; 41 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2752.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2752/head:pull/2752

PR: https://git.openjdk.java.net/jdk/pull/2752