[9.16] deprecate dscp configuration

This commit deprecates the "dscp" configuration option and "dscp" parameters to source-address configuration options (query-source, transfer-source, etc.

(Note that the DSCP feature has not been fully operational since the network manager was introduced in 9.16.0; outgoing DSCP values can be configured, but incoming DSCP values are not detected.)

Closes #3773 (closed)