tools: completed relocation of former 'xtfs_snap' functionality into 'xtfsutil'

* removed all references to 'xtfs_snap' in the source code
* updated man pages
* updated system tests
* updated user guide

git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@3527 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534