busybox

bc: change bc_read_line() and zbc_vm_stdin() to avoid double buffers

function                                             old     new   delta
bc_read_line                                         129     124      -5
bc_vm_run                                            523     433     -90
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-95)             Total: -95 bytes
   text	   data	    bss	    dec	    hex	filename
 980445	    485	   7296	 988226	  f1442	busybox_old
 980350	    485	   7296	 988131	  f13e3	busybox_unstripped

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