bc: shrink parsing code a bit more, disallow "auto a b c" (without commas)

bc: shrink parsing code a bit more, disallow "auto a b c" (without commas)

function                                             old     new   delta
bc_parse_expr_empty_ok                              1791    1785      -6
zbc_parse_stmt_possibly_auto                        1675    1599     -76
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-82)             Total: -82 bytes

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