libctf: serialize: handle CTF-versus-BTF output format checks

libctf: serialize: handle CTF-versus-BTF output format checks

The internal function ctf_serialize_output_format centralizes all the checks
for BTF-versus-CTF, checking to see if the type section, active
suppressions, and BTF-emission mode permit BTF emission, setting
ctf_serialize.cs_is_btf if we are actually BTF, and raising ECTF_NOTBTF if
we are requiring BTF emission but the type section is such that we can't
emit it.

(There is a forcing parameter in place, as with most of these serialization
functions, to allow for the caller to force CTF emission if it knows the
output will be compressed or will be part of multi-member archives or
something else external to the type section that BTF does not support.)