domain_to_str(): enforce WDNS_MAXLEN_NAME, not WDNS_PRESLEN_NAME

'src' is a wire-format domain, so it cannot be longer than
WDNS_MAXLEN_NAME bytes. (WDNS_PRESLEN_NAME is the wrong limit, it's too
much.)

also, on NameException, return the failed domain name.
reverse_name(): enforce WDNS_MAXLEN_NAME limit on input

wdns_reverse_name() requires that the output buffer cannot be smaller
than the input buffer, so enforce that the input is no longer than the
size of the output buffer (WDNS_MAXLEN_NAME).