bc: simplify, and restore ^C, fix ^D handling

bc: simplify, and restore ^C, fix ^D handling

^D used to enter infinite loop

function                                             old     new   delta
bc_read_line                                         342     359     +17
bc_args_opt                                            8       -      -8
dc_sig_msg                                            31       -     -31
bc_sig_msg                                            34       -     -34
bc_vm_run                                           2608    2569     -39
bc_args                                              123      83     -40
bc_args_lopt                                          81       -     -81
------------------------------------------------------------------------------
(add/remove: 0/4 grow/shrink: 1/2 up/down: 17/-233)          Total: -216 bytes
   text	   data	    bss	    dec	    hex	filename
 989491	    485	   7336	 997312	  f37c0	busybox_old
 989425	    485	   7336	 997246	  f377e	busybox_unstripped

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