udhcpc: kill undocumented -W, it was a no-op.

udhcpc: kill undocumented -W, it was a no-op.
 fix option parsing in the case some CONFIG_x are off.
 disable -b on NOMMU, make backgrounding work correctly
 (if a bit differently from MMU case). Previously,
 it wasn't working at all.
 stop using global data for flags in main(), opt
 bitfield works as well.

function                                             old     new   delta
cryptpw_main                                         177     153     -24
packed_usage                                       24478   24452     -26
client_background                                     26       -     -26
udhcpc_main                                         2462    2372     -90
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(add/remove: 0/1 grow/shrink: 0/3 up/down: 0/-166)           Total: -166 bytes