bc: further conversions to "z" logic

bc: further conversions to "z" logic

function                                             old     new   delta
bc_program_exec                                     3876       -   -3876
zbc_program_exec                                       -    3963   +3963
bc_program_execStr                                   429       -    -429
zbc_program_execStr                                    -     427    +427
bc_vm_file                                           197       -    -197
zbc_vm_file                                            -     187    +187
bc_vm_process                                         74       -     -74
zbc_vm_process                                         -      61     +61
bc_parse_pushIndex                                    66      68      +2
bc_vm_run                                            624     618      -6
dc_main                                              187     176     -11
bc_program_reset                                      64       -     -64
zbc_program_pushArray                                111       -    -111
------------------------------------------------------------------------------
(add/remove: 4/6 grow/shrink: 1/2 up/down: 4640/-4768)       Total: -128 bytes
   text	   data	    bss	    dec	    hex	filename
 980922	    485	   7296	 988703	  f161f	busybox_old
 980794	    485	   7296	 988575	  f159f	busybox_unstripped

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