java-info: enclose messages in single quotes

This avoids potential syntax errors.
java-info: be more flexible about JDK paths

In particular, when you install the Java sources and javadoc from
Apple's website, even though it supposed to augment the Apple Java
installed in /System/Library/Java, it *actually* installs a whole new
JDK into /Library/Java, complete with sources and javadocs; the original
JDK under /System is untouched. If you then e.g. search for JREs in
Eclipse, it will find two versions of the same Java 6 JRE under
/System/Library/Java and /Library/Java. How utterly confusing...