Add dns_fixedname_initname()

The following pattern is repeated in many places in BIND code:

dns_fixedname_t fixed;
dns_name_t *name;

dns_fixedname_init(&fixed);
name = dns_fixedname_name(&name);

Let's add a helper function that does the equivalent:

dns_fixedname_t fixed;
dns_name_t *name;

name = dns_fixedname_initname(&fixed);

Implementation would be:

dns_name_t *
dns_fixedname_initname(dns_fixedname_t *fixed) {
    dns_fixedname_init(fixed);
    return (dns_fixedname_name(fixed));
}