libctf: decl, types: revise ctf_decl*, ctf_type_*name

libctf: decl, types: revise ctf_decl*, ctf_type_*name

These all need fairly trivial revisions for prefix types.  While
we're at it, we can add explicit handling of nonrepresentable types,
returning CTF_K_UNKNOWN for such types rather than throwing an error,
so that type printing prints (nonrepresentable type) for such types
as it always intended to.