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

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

Adding this feature to lsblk(8) was a consideration, but lsblk is not
interested of swapfiles, so the swapon seems like a better option to add
this information.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
swapon: don't use empty strings for smartcols output

The libsmartcols is smart enough to accept NULL for empty fields
rather than empty strings.

Signed-off-by: Karel Zak <kzak@redhat.com>