issue separate DNS queries for ipv4 and ipv6

issue separate DNS queries for ipv4 and ipv6

Adding multiple questions on a single DNS query is not supportted by
most DNS servers. This patch issues two separate DNS queries
sequentially for ipv4 and then for ipv6.

Fixes: https://savannah.gnu.org/bugs/?39710
 * grub-core/net/bootp.c (parse_dhcp_vendor): Add DNS option.
 * grub-core/net/dns.c (grub_dns_qtype_id): New enum.
 * (grub_net_dns_lookup): Now using separated dns packages.
 * (grub_cmd_nslookup): Add error condition.
 * (grub_cmd_list_dns): Print DNS option.
 * (grub_cmd_add_dns): Add four parameters: --only-ipv4,
 * --only-ipv6, --prefer-ipv4, and --prefer-ipv6.
 * include/grub/net.h (grub_dns_option_t): New enum.
 * (grub_net_network_level_address): option added.

Also-by: Paulo Flabiano Smorigo <pfsmorigo@br.ibm.com>
Signed-off-by: Paulo Flabiano Smorigo <pfsmorigo@br.ibm.com>