RFR: 6889: Provide build script for making it easier to build JMC [v3]

[Schalli1987]

> Second try, I signed the OCA last week (also I do not have any response).
> 
> This script provides an easy way to package JMC by just calling a single command.

Schalli1987 has updated the pull request incrementally with two additional commits since the last revision:

 - fix subprocessed pid of jetty server
 - fix typos, add --run hint after --packageJmc

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Changes:
  - all: https://git.openjdk.java.net/jmc/pull/100/files
  - new: https://git.openjdk.java.net/jmc/pull/100/files/e45021b2..e051a310

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jmc&pr=100&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jmc&pr=100&range=01-02

  Stats: 6 lines in 1 file changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jmc/pull/100.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/100/head:pull/100

PR: https://git.openjdk.java.net/jmc/pull/100