swapon: share get_swap_prober() with swaplabel to print uuid and label

The swapon listing was almost complete, apart from label and uuid.  This
change add the code from swaplabel(8) to shared object file to be used
for printouts in other swap commands, such as swapon.

Unarguably lsblk(8) does great job printing uuid's for devices, but does
not include swap lables for devices, of swapfiles at all.  Neither the
swap files or loopback devices are block devices.  With that thinkin
these listings fit better to swapon than in lsblk.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>