libctf: dump: dump the header; dump enum64s; adapt to API changes

libctf: dump: dump the header; dump enum64s; adapt to API changes

A bunch of dumper changes.  Most importantly, adapt to the changes in the _f
iteration function prototypes by no longer carrying around our own cds_fp
dict pointer everywhere but just using the one we are given by the iteration
function.

But also, dump the v3 and v4/BTF headers separately, using the stored
original v3-pre-upgrade header copy if present.  The v3 dumper is not tested
yet, of course, but is more or less unchanged from the old code, so probably
nearly works.  The v4 dumper is tested.

Add enum64 support (basically just a bit of extra code to print the
signedness of enums).