path: fix win32 relative() for UNC paths

win32 normalize() will output a trailing '\' for some UNC paths. trim
them before processing

Change by @mscdex

Add basic UNC path tests to win32 relative()

PR-URL: https://github.com/nodejs/node/pull/5456


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Roman Reiss <me@silverwind.io>