libctf: don't pass errno into ctf_err_warn so often

The libctf-internal warning function ctf_err_warn() can be passed a libctf
errno as a parameter, and will add its textual errmsg form to the passed-in
error message. But if there is an error on the fp already, and this is
specifically an error and not a warning, ctf_err_warn() will print the error
out regardless: there's no need to pass in anything but 0.

There are still a lot of places where we do

ctf_err_warn (fp, 0, EFOO, ...);
return ctf_set_errno (fp, 0, EFOO);

I've left all of those alone, because fixing it makes the code a bit longer:
but fixing the cases where no return is involved and the error has just been
set on the fp itself costs nothing and reduces redundancy a bit.

libctf/

	* ctf-dedup.c (ctf_dedup_walk_output_mapping): Drop the errno arg.
	(ctf_dedup_emit): Likewise.
	(ctf_dedup_type_mapping): Likewise.
	* ctf-link.c (ctf_create_per_cu): Likewise.
	(ctf_link_deduplicating_close_inputs): Likewise.
	(ctf_link_deduplicating_one_symtypetab): Likewise.
	(ctf_link_deduplicating_per_cu): Likewise.
	* ctf-lookup.c (ctf_lookup_symbol_idx): Likewise.
	* ctf-subr.c (ctf_assert_fail_internal): Likewise.