libctf: Handle CTFv3 containers

In general, the patch adds indirection to minimize the amount of code
that needs to know about differences between v2 and v3. Specifically,
some new ctf_get_ctt_* functions are added, and new LCTF_* macros are
added to use the underlying container's version to do the right thing.

CTF containers can have parent/child relationships, wherein a type ID in
one container refers to a type in the parent. It is permitted for the
parent and child to have different versions.

MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34363