miniupnpd: fix eport selection and error handling

The find_available_eport function that was intended to check if
at least one eport is allowed for a given iaddr/iport does not
work as intended; for example, it does not properly handle rule
precedence (i.e., it considers allow rules even if they are
effectively masked by earlier deny rules), and it also does not
handle the case where no rules are specified at all (which
should default to accept in order to be consistent with
check_upnp_rule_against_permissions). The present change removes
this function and instead integrates the check into the existing
while loop that iterates over all eports.