udhcp: merge clientpacket.c into dhcpc.c

udhcp: merge clientpacket.c into dhcpc.c

function                                             old     new   delta
udhcpc_main                                         2569    2855    +286
perform_release                                      122     124      +2
client_background                                     31      32      +1
init_packet                                          156      88     -68
send_decline                                         114       -    -114
send_discover                                        121       -    -121
send_select                                          130       -    -130
------------------------------------------------------------------------------
(add/remove: 0/4 grow/shrink: 3/1 up/down: 289/-433)         Total: -144 bytes

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