bc: stop propagating pointer to G.prog.nchars deep into printing functions

bc: stop propagating pointer to G.prog.nchars deep into printing functions

function                                             old     new   delta
bc_num_printHex                                       62      71      +9
bc_num_printNewline                                   47      54      +7
bc_num_printDigits                                   132     136      +4
bc_num_printChar                                      23      27      +4
bc_program_printStream                               173     163     -10
bc_program_print                                     754     739     -15
bc_num_printNum                                      538     514     -24
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 4/3 up/down: 24/-49)            Total: -25 bytes

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