RFR: JDK-8209963: source file mode for JVM should provide a hook to locate the source file

[Jonathan Gibbons]

Please review a 1-line patch, and test, for source-file mode,
to make the name of the source file available in a system property.

This patch is the last in the recent series of minor enhancements to
the source-file launcher feature.

JBS: https://bugs.openjdk.java.net/browse/JDK-8209963
CSR:https://bugs.openjdk.java.net/browse/JDK-8210877
Webrev: http://cr.openjdk.java.net/~jjg/8209963/webrev.00/index.html

-- Jon