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.
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34364