libctf: create: DTD addition and deletion; ctf_rollback

libctf: create: DTD addition and deletion; ctf_rollback

DTD deletion changes mostly relate to the changes to the ctf_dtdef_t, but
also we no longer nede to have special handling for forwards (we can just
use ctf_type_kind_forwarded like everyone else).

Rollback no longer needs to delete things by hand (it hasn't needed to for
years): it can just call ctf_dtd_delete.

ctf_add_generic changes substantially, mostly to allow for the ctf_dtdef_t
changes.  Rather than returning a type ID it now returns the DTD it just
allocated: it can also be asked to add some prefixes, and return the first
prefix added (which may not be the first prefix in the type, because if it
is asked to add a non-root-visible type it will additionally allocate a
CTF_K_CONFLICTING prefix to encode that).

Finally, duplicate name detection is suppressed for type and decl tags.