sfdisk: make some tests conditional to !Linux

sfdisk: make some tests conditional to !Linux

This patch makes the following tests/actions conditional to "!Linux":

- Force cylinders as format instead of MB, even if user asked for MB.
  This solves a bug where if you use "-L -uM", set 1 as starting MB
  and the disk is larger than a certain size (about 1GB) the partition
  would start at sector 1 instead of 1MB due to cyl rounding.

- Warn about partitions not starting/ending on cyl boundaries.

- Check if CHS is ok.

I used the "!Linux" notation since it was already used elsewhere in
the code.

Signed-off-by: Giulio Orsero <giulioo@pobox.com>
Signed-off-by: Karel Zak <kzak@redhat.com>