udhcpc: make it possible to disable vendor id; improve help text

udhcpc: make it possible to disable vendor id; improve help text

function                                             old     new   delta
init_packet                                          135     139      +4
packed_usage                                       26789   26786      -3
alloc_dhcp_option                                     67      63      -4
udhcpc_main                                         2467    2447     -20
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(add/remove: 0/0 grow/shrink: 1/3 up/down: 4/-27)             Total: -23 bytes

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