chsh, chfn, vipw: fix filenames collision

The utils when compiled WITHOUT libuser then mkostemp()ing
"/etc/%s.XXXXXX" where the filename prefix is argv[0] basename.

An attacker could repeatedly execute the util with modified argv[0]
and after many many attempts mkostemp() may generate suffix which
makes sense. The result maybe temporary file with name like rc.status
ld.so.preload or krb5.keytab, etc.

Note that distros usually use libuser based ch{sh,fn} or stuff from
shadow-utils.

It's probably very minor security bug.

Addresses: CVE-2015-5224
Signed-off-by: Karel Zak <kzak@redhat.com>
static char *find_field(char *nf, char *of)
{
	if (nf)
		return nf;
	if (of)
		return of;
	return xstrdup("");
}

static char *find_field(char *nf, char *of)
{
	if (nf)
		return nf;
	if (of)
		return of;
	return xstrdup("");
}
