linux-user: add SIOCADDRT/SIOCDELRT support

linux-user: add SIOCADDRT/SIOCDELRT support

This allows to pass the device name.

You can test this with the "route" command.

WITHOUT this patch:

$ sudo route add -net default gw 10.0.3.1 eth0
SIOCADDRT: Bad address
$ netstat -nr
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Ifa
10.0.3.0        0.0.0.0         255.255.255.0   U         0 0          0 eth

WITH this patch:

$ sudo route add -net default gw 10.0.3.1 eth0
$ netstat -nr
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Ifa
0.0.0.0         10.0.3.1        0.0.0.0         UG        0 0          0 eth
10.0.3.0        0.0.0.0         255.255.255.0   U         0 0          0 eth

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>