Changing basefile to use path-platform's split.

Currently manta-sync will file.substr(localdir.length) leaving a basefile like `'\foo.txt'`, then later when you have the posix join you get a path like `'/mantauser/stor/\foo.txt'`

This change chops off the leading `/ or \` 
Oops you are right, I just grab the last part of the path.  You need the localpath plus any subdirectories.