This commit fixes loadufs command line to take parameters: bin/tachyon l...

This pull request fixes the loadufs command line to take two or three parameters: TachyonPath UnderfsPath OptionalExcludingPrefixes

(1) It adds a new method UnderfsUtil.loadUnderFs/5 for implementing the new loadufs command line
     while keeping the old loadUnderFs/4 for compatibility.
(2) It adds toString methods in PrefixList and TachyonFS for printing meaningful debug messages.
(3) It fixes list() in UnderFileSystem classes to return relative paths, to be consistent with java.io.File.
(4) It smooths out the uses of list() in other modules.
(5) It fixes docs/Syncing-the-Underlying-Filesystem.md accordingly.

The code has been tested on Mac local file system. Currently the path names cannot have spaces, which could be fixed later.