[9.18] Merge branch '3790-use-configured-udp-port' into 'main'

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.)

(cherry picked from commit 4d50c912)

Backport of !7479 (merged)