bc: fixes for multi-line if/while/for

bc: fixes for multi-line if/while/for

function                                             old     new   delta
zbc_vm_process                                       561     589     +28
zbc_lex_next_and_skip_NLINE                            -      22     +22
zbc_parse_stmt_possibly_auto                        2232    2253     +21
zbc_lex_skip_if_at_NLINE                               -      14     +14
zbc_lex_number                                       192     200      +8
zbc_num_divmod                                       150     156      +6
bc_vm_run                                            134     139      +5
bc_vm_init                                           757     760      +3
bc_num_printNewline                                   51      54      +3
------------------------------------------------------------------------------
(add/remove: 2/0 grow/shrink: 7/0 up/down: 110/0)             Total: 110 bytes
   text	   data	    bss	    dec	    hex	filename
 982138	    485	   7296	 989919	  f1adf	busybox_old
 982275	    485	   7296	 990056	  f1b68	busybox_unstripped

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