handle swap partitions better during import/export

handle swap partitions better during import/export

Changes the export file format so that the first
line indicates whether the device being exported or
imported is a swap device or an ext* filesystem.

If it's a swap device, it's just recreated on import;
there's no reason to retain its contents.

WARNING: This makes previous gnt-backup filesystem exports
incompatible unless they are modified so that their first
line contains "ext3" or "ext4".

Signed-off-by: Galen Charlton <gmc@esilibrary.com>