[9.18] mark "port" as deprecated for source address options

Backport of !7307 (merged)

This backport also deprecates setting port for alt-transfer-source(-v6) (because it is still present in v9_18) and adds a hunk that sets CFG_ADDR_PORTOK for sockaddrdscp_flags (without this, the forwarders statement on v9_18 would not accept the port argument and that is not what we did in main, where there is no DSCP-related code any more).

Closes #3781 (closed)