udhcpc: make -O <numeric_opt> work. Closes 5402

udhcpc: make -O <numeric_opt> work. Closes 5402

function                                             old     new   delta
udhcpc_main                                         2642    2685     +43
udhcp_recv_raw_packet                                414     415      +1
d6_recv_raw_packet                                   248     249      +1
udhcpc6_main                                        2430    2413     -17
add_client_options                                   239     213     -26
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/2 up/down: 45/-43)              Total: 2 bytes

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