ctf: Add v3 support to CTF tools, ctf{convert,dump,merge}

ctfdump handles v2 and v3. ctfconvert now emits only CTFv3, whereas
ctfmerge can merge v2 and v3 containers into v3 containers.

Approved by: re (gjb)
Sponsored by: The FreeBSD Foundation

(cherry picked from commit bdf290cd3e1a69d41c2f8bb60bd415cfa78adba2)
(cherry picked from commit e5b54808f735f1a74716f1320916b8a961ddf92d)