RFR (M) 8191521: handle long relative path specified in -Xbootclasspath/a on windows

[Schmelter, Ralf]

Hi Calvin,

I've updated the webrev with your suggestions. The tests now disable the
UNC path portion, if you don't have share <DRIVE>$ for <DRIVE>: 
So all tests should now run on your machine.

http://cr.openjdk.java.net/~rschmelter/webrevs/8191521/webrev.1/

Best regards,
Ralf