linux-user: add rtnetlink(7) support

rtnetlink is needed to use iproute package (ip addr, ip route)
and dhcp client.

Examples:

Without this patch:
    # ip link
    Cannot open netlink socket: Address family not supported by protocol
    # ip addr
    Cannot open netlink socket: Address family not supported by protocol
    # ip route
    Cannot open netlink socket: Address family not supported by protocol
    # dhclient eth0
    Cannot open netlink socket: Address family not supported by protocol
    Cannot open netlink socket: Address family not supported by protocol

With this patch:
    # ip link
    1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT
        link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    51: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP mode DEFAULT qlen 1000
        link/ether 00:16:3e:89:6b:d7 brd ff:ff:ff:ff:ff:ff
    # ip addr show eth0
    51: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP qlen 1000
        link/ether 00:16:3e:89:6b:d7 brd ff:ff:ff:ff:ff:ff
        inet 192.168.122.197/24 brd 192.168.122.255 scope global eth0
           valid_lft forever preferred_lft forever
        inet6 fe80::216:3eff:fe89:6bd7/64 scope link
           valid_lft forever preferred_lft forever
    # ip route
    default via 192.168.122.1 dev eth0
    192.168.122.0/24 dev eth0  proto kernel  scope link  src 192.168.122.197
    # ip addr flush eth0
    # ip addr add 192.168.122.10 dev eth0
    # ip addr show eth0
    51: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP qlen 1000
        link/ether 00:16:3e:89:6b:d7 brd ff:ff:ff:ff:ff:ff
        inet 192.168.122.10/32 scope global eth0
           valid_lft forever preferred_lft forever
    # ip route add 192.168.122.0/24 via 192.168.122.10
    # ip route
        192.168.122.0/24 via 192.168.122.10 dev eth0

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>