Disallow explicit small payload offset for detached header.

Disallow explicit small payload offset for detached header.

LUKS detached header has some limitations, one of them
is that you cannot run some explicit check for data offsets
without providing also data device.

Because luksDump and all key handle commands takes only
metadata device (LUKS heaer device), it not easy to properly
support data payload offset validation.

So if detached header is present for luksFormat, code now
allows data payload 0 (IOW whole data device is used)
and explicit offset larger than header+keyslots
(the same as the header is on data device - so some space is wasted).

N.B. with detached header the option --align-payload is used
directly without any round up caculations.

Fixes Issue#155.