Fix issue #764: support multiple payloads per UDP port

[bogi788]

This MR adds support for sending multiple payloads per UDP port.

For this it:

• modifies payloads_datagram_add to allow multiple payloads per port
• adds a new "port type" for UDP payloads (instead of actual ports)
• creates a targets list in transmit_thread than can reference UDP payloads directly
• updates template_set_target_ipv4 and template_set_target_ipv6 to handle these UDP payloads

Due to these changes template_set_target_ipv4 and template_set_target_ipv6 are only called for UDP ports (rather than payloads) if no specialised payload was found.