bc: trim banner, use empty interactive prompts (GNU bc compat)

bc: trim banner, use empty interactive prompts (GNU bc compat)

function                                             old     new   delta
bc_vm_run                                            689     684      -5
bc_program_read                                      355     350      -5
bc_read_line                                         307     285     -22
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/3 up/down: 0/-32)             Total: -32 bytes
   text	   data	    bss	    dec	    hex	filename
 985409	    485	   7296	 993190	  f27a6	busybox_old
 985265	    485	   7296	 993046	  f2716	busybox_unstripped

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