8062955: (fs spec) Files.setLastModifiedTime should specify SecurityException more clearer

[Alan Bateman]

This issue is a complaint about the wording of the SecurityException 
that Files.setLastModifiedTime can throw, also a complaint that the 
@return doesn't make it clear that it's the given path that is returned 
(as this method doesn't have an obvious return value). While looking at 
this method I see that we don't have any test coverage so I've added a 
test for it (the underlying implementation is well covered, it's just 
the one-line method in Files that wasn't tested). The webrev with the 
proposed changes is here:
   http://cr.openjdk.java.net/~alanb/8062955/webrev/

Thanks,

Alan