bc: prepare for char-by-char input handling

bc: prepare for char-by-char input handling

function                                             old     new   delta
peek_inbuf                                             -     292    +292
parse_lex_by_checking_eq_sign                          -      26     +26
eat_inbuf                                              -      22     +22
zbc_vm_execute_FILE                                   52      61      +9
bc_lex_lineComment                                    29      30      +1
zbc_lex_number                                       174     172      -2
bc_vm_run                                            104      99      -5
zbc_num_divmod                                       156     150      -6
bc_lex_file                                           24       -     -24
bc_lex_assign                                         26       -     -26
zbc_lex_next                                        1982    1587    -395
------------------------------------------------------------------------------
(add/remove: 3/2 grow/shrink: 2/4 up/down: 350/-458)         Total: -108 bytes

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