gpt: Fixed off by one error.

* The backup header would have been written one block behind the actual
last block. Well, at least it would have tried to do so :-)

gpt: Always use at least 4K to align partitions.

* Many of today's disks that use 4K internally don't advertize this.

gpt: Pointer style changed to preferred.

gpt: Fixed a number of remaining issues.

* The header and table is now correctly written; the backup is still
missing, though.
* The Header class is now responsible for both, the primary, and the
backup header.
* Changed the Header constructors: the block is no longer needed. Also,
under GCC 4 the initialization code accidentally used the read
Header constructor.
* Fixed incorrectly copied GUID - the static_guid cannot be copied
into a guid_t directly.
* Fixed copy&paste bug that would overwrite the offset for the child
partition to be created.
* With all of this in place I successfully created a BFS partition
with a GUID partition table. However, I have not yet tested if other
systems can still read this. Also, creating two partitions doesn't
seem to work yet, either (luckily I only need a single one ;-)).

Implemented write support, reorganized source files.

* Moved some functionality into their own files so that they can easily
be reused by other code.
* Added crc32() function from FreeBSD. Implemented CRC handling and
validation.
* Implemented missing write functionality.

gpt: do not advertize the partition as read-only.

Renamed partitioning system "efi" directory to "gpt".