libctf: dump header offsets into the debugging output

libctf: dump header offsets into the debugging output

This is an essential first piece of info needed to debug both libctf
writing and reading problems, and we weren't recording it anywhere!

(This is a short-term fix: fairly soon, we will record all of this in a
form that outlives ctf_bufopen, and then ctf_dump() will be able to dump
it like it can everything else.)

libctf/
	* ctf-open.c (ctf_bufopen): Dump header offsets into the debugging
	output.