In order to allow mkimg(1) (and other tools) to become a build tool
that can be compiled on various OSes (including on older versions
of FreeBSD), make it possible to have it include the partitioning
scheme definitions without pulling in FreeBSD specifics.
In particular this means:
o move the scheme definitions iand related defines to header files

under sys/disk,

o make them (more) portable by using uint#_t (where applicable)

and renaming defines so that they at least have a good prefix,

o make the new headers stand-alone so that they don't need FreeBSD

definitions, like struct uuid(*)

o keep the original headers for compatibility, but rewrite them to

get the scheme definitions from <sys/disk/$scheme.h>.

(*) since UUID/GUID type definitions are non-portable and the GPT
scheme uses them, make it possible to have the scheme definitions
use an external type by allowing consumers of the header to set
GPT_UUID_TYPE. When GPT_UUID_TYPE has not been defined, the header
will use it's own type definition, which is the same as struct uuid.
The gpt_uuid_t typedef is created to abstract the details and allows
consumers to refer to a single type.

There is not conflict between the partitioning scheme headers and
what is defined in them. All headers can be included in the same
source files.

Note: consumers of the old headers have not been changed yet. Such
will be done if and when needed/beneficial.

Reviewed by: imp, jhb
MFC after: 1 month
Sponsored by: Bracket Computing