Allow saving to/restoring from a file without shell redirection

Allow saving to/restoring from a file without shell redirection

Mathieu Bridon suggested that in some environments where there is no
access to a full shell with input/output redirection, it'd be useful
to read from/write to directly a file (bugzilla #788).

The patch adds the new "-file" option to specify a filename to print
into when listing/saving sets or read from when restoring sets.