CMFileManager: Fix mv operation on Cross-device link failure

Issue https://github.com/jruesga/CMFileManager/issues/31.

The busybox mv command returns a failure when files are moved between filesystem (Cross-device
link failure). This changes fix this failure by:

 * Replace mv command for a "mv or cp+rm" operation.
 * rm operation occurs over source mount point so this need to be mounted prior to execute this
   new command.
 * Fix the stderr buffer filling on error.

Change-Id: If113ad810e7daffeb8379589789b2855eda14908