disk partition alignment

Cheap flash drives need partitions aligned to 4 MiB in order to not be slow
(and to avoid extra writes). <https://lwn.net/Articles/428584/>
And at least 1 MiB alignment is generally a good idea, and most people
seem to think 4 MiB is for all drives.

I noticed that Parted.partitioned does not do that; the first
partition started at an offset of 1 MB, and subsequent partitions
from where it ends. (The 1 MB offset came from the PartedVal PartSize
instance, and note that it was not 1 MiB.)

* Parted: Add an Alignment parameter. (API change)
  A good default to use is safeAlignment, which is 4MiB,
  well suited for inexpensive flash drives, and fine for other disks too.
  Previously, a very non-optimial 1MB (not 1MiB) alignment had been used.
* DiskImage: Use safeAlignment. It didn't seem worth making the
  alignment configurable here.

Alignment is implemented by offsetting the first partition's start
position so it's aligned (making sure to leave room for the partition
table). Each partition is then extended as needed so the next partition
will start properly aligned.

Note that parted rejects partition tables that don't fit in cylinder
bounderies. Before, propellor let parted deal with the fine details
of layout, so that was not a problem. Now it's possible to set some
wacky Alignment not divisible by 512, or use Byte sizes for partitions
and create a partition table that parted rejects. But, using
safeAlignment and MegaBytes should always be safe.

Also, this fixes a rounding bug in Parted.calcPartTable.
It was rounding up to the nearest MegaByte when allocating remaining
disk space, so returned partition table that was actually larger than
the disk size.

This commit was sponsored by an anonymous bitcoiner.

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