udhcpc: paranoia when using kernel UDP mode for sending renew: server ID may be bogus

udhcpc: paranoia when using kernel UDP mode for sending renew: server ID may be bogus

With new code, we request that target IP (server ID) must be directly reachable.
If it's not, this happens:

udhcpc: waiting 2000 seconds
udhcpc: entering listen mode: kernel
udhcpc: opening listen socket on *:68 wlan0
udhcpc: entering renew state
udhcpc: sending renew to 1.1.1.1
udhcpc: send: Network is unreachable
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 1.1.1.1 needs routing, this is fishy!
udhcpc: entering rebinding state
udhcpc: entering listen mode: raw
udhcpc: created raw socket
udhcpc: sending renew to 0.0.0.0
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ going to use broadcast

which is the desired behavior. Before the patch, packet to 1.1.1.1 was routed
over eth0 (!) and maybe even into Internet (!!!).

function                                             old     new   delta
udhcpc_main                                         2752    2763     +11
udhcp_send_kernel_packet                             295     301      +6
send_renew                                            82      84      +2
send_packet                                          166     168      +2
bcast_or_ucast                                        23      25      +2
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 5/0 up/down: 23/0)               Total: 23 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>