dhcp: heed TODO item - divorced options from their string descriptions

dhcp: heed TODO item - divorced options from their string descriptions
code shrink while at it.

function                                             old     new   delta
dhcp_option_strings                                    -     258    +258
udhcp_run_script                                    1135    1174     +39
dhcp_option_lengths                                    -      11     +11
udhcp_add_simple_option                               93      92      -1
packet_num                                             4       -      -4
read_opt                                             746     739      -7
udhcp_option_lengths                                  11       -     -11
udhcpc_main                                         2590    2494     -96
dhcp_options                                         490      70    -420
------------------------------------------------------------------------------
(add/remove: 2/2 grow/shrink: 1/4 up/down: 308/-539)         Total: -231 bytes
   text    data     bss     dec     hex filename
 775309     929    9100  785338   bfbba busybox_old
 775098     929    9084  785111   bfad7 busybox_unstripped