RFR 8189777: jlink --module-path default value and automatic addition of $JAVA_HOME/jmods if java.base is missing

[Sundararajan Athijegannathan]

Please review.

Bug: https://bugs.openjdk.java.net/browse/JDK-8189777
Webrev: http://cr.openjdk.java.net/~sundar/8189777/webrev.00/

Thanks,
-Sundar