bc: optimize non-interactive config: we know that many functions always succeed

bc: optimize non-interactive config: we know that many functions always succeed

function                                             old     new   delta
bc_program_assign                                    475     471      -4
bc_program_prep                                       87      81      -6
bc_parse_parse                                       423     417      -6
bc_num_p                                             440     434      -6
bc_program_printStream                               153     143     -10
bc_program_execStr                                   478     468     -10
bc_parse_auto                                        275     265     -10
bc_num_k                                             925     913     -12
bc_program_copyToVar                                 307     289     -18
bc_program_binOpPrep                                 289     271     -18
bc_num_printNum                                      489     471     -18
bc_num_m                                             287     269     -18
bc_program_pushArray                                 139     113     -26
bc_program_modexp                                    707     677     -30
bc_program_print                                     704     667     -37
bc_program_exec                                     4010    3920     -90
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/16 up/down: 0/-319)          Total: -319 bytes
   text	   data	    bss	    dec	    hex	filename
 983202	    485	   7296	 990983	  f1f07	busybox_old
 982883	    485	   7296	 990664	  f1dc8	busybox_unstripped

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