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.

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

(cherry picked from commit a6fb86917362e3f6d24e95e940e80845c2cfde8a)
(cherry picked from commit a49aa9da7835742aa455c7eec4493f0034b4e197)