parse-helpers: allow port 0 for socket bind items

This patch adds a new parameter to parse_ip_port_range, giving callers
the option to allow ranges to have their min be 0 instead of 1.

This is then used by parse_ip_ports_token, intern used by
parse_socket_bind_item to allow port 0 when restricting bind system
calls with SocketBindDeny / SocketBindAllow.

With this, users running server software written using the golang
standard library will be able to effectively sandbox their software,
albeit with a small loss in security protections by allowing the
process to bind on a random port in the
/proc/sys/net/ipv4/ip_local_port_range.

src/network/netdev/vxlan.c

@@ -289,7 +289,7 @@ int config_parse_port_range(
         VxLan *v = ASSERT_PTR(userdata);
         int r;
 
-        r = parse_ip_port_range(rvalue, &v->port_range.low, &v->port_range.high);
+        r = parse_ip_port_range(rvalue, &v->port_range.low, &v->port_range.high, /* allow_zero = */ false);
         if (r < 0)
                 log_syntax(unit, LOG_WARNING, filename, line, r,
                            "Failed to parse VXLAN port range '%s'. Port should be greater than 0 and less than 65535.", rvalue);

src/network/networkd-routing-policy-rule.c

@@ -1408,7 +1408,7 @@ int config_parse_routing_policy_rule_port_range(
         if (r < 0)
                 return log_oom();
 
-        r = parse_ip_port_range(rvalue, &low, &high);
+        r = parse_ip_port_range(rvalue, &low, &high, /* allow_zero = */ false);
         if (r < 0) {
                 log_syntax(unit, LOG_WARNING, filename, line, r, "Failed to parse routing policy rule port range '%s'", rvalue);
                 return 0;