RFR 8211941 : Enable checking/ignoring of non-conforming Class-Path entries

[Brent Christian]

Hi,

JDK-8216401[1][2] added code to improve JAR spec conformance (WRT 
relative URLs in the Class-Path attribute), while maintaining some key 
compatibility aspects (entries using "file:" URLs are not relative, but 
are used by some libraries).

This code was disabled by default, but we would now like to enable it.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8211941
Webrev:
http://cr.openjdk.java.net/~bchristi/8211941/webrev-06/

This change includes a new system property 
("jdk.net.URLClassPath.showIgnoredClassPathEntries") that can be set to 
aid in debugging JAR Class-Paths by printing entries that are ignored.

Thanks,
-Brent

1. https://bugs.openjdk.java.net/browse/JDK-8216401

2. 
https://mail.openjdk.java.net/pipermail/core-libs-dev/2019-January/057868.html