Add overwrite control to bup-restore

Adds the --overwrite-files and --directory-merge options to bup restore.
These control whether restore will overwrite files, or modify directory
structures, of any files or directories which exist in the restore
location.

Outstanding issue:
	- raw_input headers don't print till after the selection is made.

Signed-off-by: Will Rouesnel <w.rouesnel@gmail.com>