RFR(JDK11/NIO) 8202285: (fs) Add a method to Files for comparing file contents

[Joe Wang]

Hi,

Considering extending isSameFile to add isSameContent to Files. Please 
review.

JBS: https://bugs.openjdk.java.net/browse/JDK-8202285

webrev: http://cr.openjdk.java.net/~joehw/jdk11/8202285/webrev/

specdiff: 
http://cr.openjdk.java.net/~joehw/jdk11/8202285/specdiff/java/nio/file/Files.html

Thanks,
Joe