Recognise when Linux swap label is changed to blank (#685656)

Use swaplabel to read the Linux swap label so that 1) a blank label can
be read, and 2) to replace deprecated and removed vol_id.

Vol_id was deprecated in May 2009, superseded by blkid and soon
thereafter removed.  Swaplabel is available in util-linux 2.18 from Jun
2010.

If the swaplabel command is not available GParted falls back on reading
the label through the FS_Info cache which uses the blkid command.  If
the blkid command is also not available FS_Info cache will *NOT* return
any label and GParted will display blank.

Bug 685656 - GParted doesn't notice when file system label is changed to
             blank