utils.c: Add ip address validation

utils.c: Add ip address validation

Do not allow configuring invalid IPv4/IPv6 addresses.
Curently if I configure LAN IP Address as 224.1.1.1, netifd will
configure it.
e.g.
uci set network.lan.ipaddr='224.1.1.1'
uci commit
/etc/init.d/network restart

Now ifconfig br-lan returns
br-lan    Link encap:Ethernet  HWaddr 00:03:7F:13:BA:17
          inet addr:224.1.1.1  Bcast:224.1.1.255  Mask:255.255.255.0
which is wrong.

If I use ifconfig eth1 224.1.1.1, I will get
ifconfig: SIOCSIFADDR: Invalid argument

it means ifconfig is working fine, whereas netifd not.
Proposed patch will test IPv4 address to make sure it is class A/B/C only.
Similarly IPv6 multicast addresses will not be allowed.

Signed-off-by: Naresh Kumar Mehta <naresh@codeaurora.org>