nslookup: rework option parsing

nslookup: rework option parsing

function                                             old     new   delta
nslookup_main                                       2715    2754     +39
packed_usage                                       32179   32211     +32
add_ns                                                65      66      +1
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/0 up/down: 72/0)               Total: 72 bytes
   text	   data	    bss	    dec	    hex	filename
 926262	    555	   5740	 932557	  e3acd	busybox_old
 926239	    555	   5740	 932534	  e3ab6	busybox_unstripped

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