bc: for "dc only" remove handling of LAST, move OBASE enums up to IBASE

bc: for "dc only" remove handling of LAST, move OBASE enums up to IBASE

function                                             old     new   delta
dc_LEX_to_INST                                         -      82     +82
bc_parse_pushNUM                                       -      62     +62
zbc_program_num                                      981     978      -3
bc_result_free                                        46      43      -3
zbc_program_binOpPrep                                305     300      -5
static.msg                                            24      12     -12
zbc_program_exec                                    4013    3994     -19
zdc_parse_expr                                       583     507     -76
dc_parse_insts                                        83       -     -83
------------------------------------------------------------------------------
(add/remove: 2/1 grow/shrink: 0/6 up/down: 144/-201)          Total: -57 bytes

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