libctf: drop LCTF_TYPE_ISPARENT/LCTF_TYPE_ISCHILD

libctf: drop LCTF_TYPE_ISPARENT/LCTF_TYPE_ISCHILD

Parent/child determination is about to become rather more complex, making a
macro impractical.  Use the ctf_type_isparent/ischild function calls
everywhere and remove the macro.  Make them more const-correct too, to
make them more widely usable.

While we're about it, change several places that hand-implemented
ctf_get_dict() to call it instead, and armour several functions against
the null returns that were always possible in this case (but previously
unprotected-against).