disklabel.py: Print BSD->Linux partition mapping

Add an input option in disklabel.py to print the partition mapping the
Linux kernel will do on the disklabel.
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 device.
disklabel.py: Add function for printing warnings
disklabel: When enlarging round up OpenBSD parts

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