bc: POSIX error/warn functions can be 'z' functions too

bc: POSIX error/warn functions can be 'z' functions too

In non-interactive config, they either return 'success', or do not return.

function                                             old     new   delta
zbc_posix_error_fmt                                   41      39      -2
bc_parse_expr_empty_ok                              1751    1744      -7
zbc_parse_stmt_possibly_auto                        1322    1314      -8
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(add/remove: 5/5 grow/shrink: 0/2 up/down: 118/-135)          Total: -17 bytes

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