bc: hook up line editing with history buffer

bc: hook up line editing with history buffer

function                                             old     new   delta
push_input_byte                                        -      65     +65
bc_vm_run                                           1875    1905     +30
bc_read_line                                         303     305      +2
bc_num_binary                                        148     150      +2
bc_num_ulong                                         103      92     -11
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(add/remove: 1/0 grow/shrink: 3/1 up/down: 99/-11)             Total: 88 bytes

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