libctf: open, types: ctf_import for BTF

libctf: open, types: ctf_import for BTF

ctf_import needs a bunch of fixes to work with pure BTF dicts -- and, for
that matter, importing newly-created parent dicts that have never been
written out, which may have a bunch of nonprovisional types (if types were
added to it before any imports were done) or may not (if at least one
ctf_import into it was done before any types were added).

So we adjust things so that the values that are checked against are the
nonprovisional-types values: the header revisions actually changed the name
of cth_parent_typemax to cth_parent_ntypes to make this clearer, so catch up
with that.  In the parent, we have to use ctf_idmax, not ctf_typemax.

One thing we must prohibit is that you cannot add a bunch of types to a
child and then import a parent into it: the type IDs will all be wrong
and the string offsets more so.  This was partly prohibited: prohibit it
entirely (excepting only that the not-actually-written-out void type
we might add to new BTF dicts does not influence this check).

Since BTF children don't have a cth_parent_ntypes or a cth_parent_strlen, we
cannot check this stuff, but just set them and hope.