Review Request: JDK-8162401 Support multiple --add-exports and --add-reads with the same module/package

[Mandy Chung]

http://cr.openjdk.java.net/~mchung/jdk9/webrevs/8162401/webrev.00/

Currently, `--add-exports` and `--add-reads` options do not accept duplicated module/package and it is an error to attempt to export the same package more than once on the command-line.  There are cases where allow augmenting the command-line options to add `--add-exports` or `--add-reads`.  jtreg is one example that currently parses the command-line options and has to merge properly if there exists a `--add-exports` option of a given module/package.

This patch relaxes this restriction and allow multiple `--add-exports` and `--add-reads` options with the same module/package.  In addition, these 2 options allow unknown module/package and emit a warning rather than as a fatal error, to be consistent with javac.

Mandy