bc: correctly parse dc options

bc: correctly parse dc options

function                                             old     new   delta
bc_vm_init                                             -     821    +821
bc_vm_file                                             -     226    +226
dc_main                                               41     187    +146
bc_main                                               41      73     +32
packed_usage                                       33076   33059     -17
bc_vm_run                                           1903     701   -1202
------------------------------------------------------------------------------
(add/remove: 2/0 grow/shrink: 2/2 up/down: 1225/-1219)          Total: 6 bytes
   text	   data	    bss	    dec	    hex	filename
 987046	    485	   7296	 994827	  f2e0b	busybox_old
 987037	    485	   7296	 994818	  f2e02	busybox_unstripped

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