gpt: for now, always add partitions at the end.
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 ;-)).
gpt: Pointer style changed to preferred.