use configured source ports for UDP requests

the optional 'port' option, when used with notify-source, transfer-source, etc, is used to set up UDP dispatches with a particular source port, but when the actual UDP connection was established the port would be overridden with a random one. this has been fixed.

(configuring source ports is deprecated in 9.20 and slated for removal in 9.22, but should still work correctly until then.)

Closes #3790 (closed)