libctf: Sanitize error types for PR 30836

Made sure there is no implicit conversion between signed and unsigned
return value for functions setting the ctf_errno value.
An example of the problem is that in ctf_member_next, the "offset" value
is either 0L or (ctf_id_t)-1L, but it should have been 0L or -1L.
The issue was discovered while building a 64 bit ld binary to be
executed on the Windows platform.
Example object file that demonstrates the issue is attached in the PR.

libctf/
	Affected functions adjusted.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>
Co-Authored-By: Yvan ROUX <yvan.roux@foss.st.com>

libctf/ctf-util.c

@@ -255,16 +255,6 @@ ctf_set_open_errno (int *errp, int error)
   return NULL;
 }
 
-/* Store the specified error code into the CTF dict, and then return CTF_ERR /
-   -1 for the benefit of the caller. */
-
-unsigned long
-ctf_set_errno (ctf_dict_t *fp, int err)
-{
-  fp->ctf_errno = err;
-  return CTF_ERR;
-}
-
 /* Create a ctf_next_t.  */
 
 ctf_next_t *