Fix ssh_args implementation

Previously, quotation marks were placed around the --rsh argument. While
this is necessary on the command-line to prevent the shell from
tokenizing the ssh_args, it is not necessary for perl exec from array
form and indeed, breaks rsync's tokenizing of the arguements.

This was discovered in http://sourceforge.net/p/rsnapshot/mailman/rsnapshot-discuss/thread/CAFyR6OntWmuxJ1XsUhzHuP4XFE0xMaUz8vNXHgqDgNWK4DG8ZA@mail.gmail.com/