o Provide a private definition for UUIDs (mkimg_uuid_t) because
UUIDs are not portable.
o Move mkimg_uuid() to a new file and merge both gpt_uuid_enc()
and vhd_uuid_enc() into a single mkimg_uuid_enc() that lives in the same file.
o Move the OS-specific implementation of generating a UUID to
osdep_uuidgen() and provide the implementations for FreeBSD, macOS and Linux.
o Expect the partitioning scheme headers to be found by having
a search to the directory in which the headers live. This avoids conflicts on non-FreeBSD machines.