Fix BSD to Linux partition mapping

The mapping we used would fail in some cases. If a BSD partition
has the same boundaries with an MBR one, the kernel will not map it
on a different block device. As a result, partition 'a' is sometimes
mapped on /dev/sda1 and sometimes on /dev/sda5.
disklabel.py: Add function for printing warnings
disklabel.py: Fix OpenBSD part end when enlarging

When enlarging OpenBSD partitions, round them up to multiples of 16512
which is the default fs blocksize. This is what OpenBSD's disklabel
does when creating partitions.
Support NetBSD in FilesystemResizeUnmounted task

Make FilesystemResizeUnmounted task resize the UFS file system of NetBSD
systems