dhcp: get rid of last global data

dhcp: get rid of last global data

function                                             old     new   delta
udhcpc_main                                         2680    2684      +4
state                                                  1       -      -1
listen_mode                                            1       -      -1
sockfd                                                 4       -      -4
------------------------------------------------------------------------------
(add/remove: 0/3 grow/shrink: 1/0 up/down: 4/-6)               Total: -2 bytes

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