RFR (XS): 8140683: Deduplicate Strings in UnixFileSystem canonical caches

[Claes Redestad]

Hi,

while investigating startup and footprint for jigsaw, I found this 
unrelated oddity where java.io.UnixFileSystem is missing an opportunity 
to deduplicate file paths determined to be equal when putting them into 
its internal cache. Also avoid some recalculation of paths determined to 
be canonical for no good reason and we're golden:

webrev: http://cr.openjdk.java.net/~redestad/8140683/webrev.01
bug: https://bugs.openjdk.java.net/browse/JDK-8140683

/Claes