mkswap: wipe old signature if there is no part table by default

mkswap: wipe old signature if there is no part table by default

Historically mkswap avoids wiping any signature on "whole disk",
until force option is given.

While the idea is that it should not wipe possible boot loader,
in reality it leads to many situations where e.g. LUKS device
is overwritten but still can be detected as LUKS (but unusable).

Patch chnges behaviour that only if partition table is detected,
signatures are not wiped.

Also it removes check for block device - loop device can now
map partitions in-kernel, so using mkswap on disk image in file
should behave the same as on disk.

Also it adds warning that know signature was wiped.

Signed-off-by: Milan Broz <mbroz@redhat.com>