Fix an issue where the postexec cmd script cannot unmount the USB backup...

Fix an issue where the postexec cmd script cannot unmount the USB backup drive because rsnapshot's current directory is the backup drive root.

Stolen directly from here:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=660372
http://bugs.debian.org/cgi-bin/bugreport.cgi?msg=10;filename=rsnapshot_unmount-on-cmd-postexec.patch;att=1;bug=660372

I tested and it works.

Another solution is to allow the user to specify a working directory for the post exec script.