MINOR: applet/stconn: Add a SE flag to specify an endpoint does not expect data

MINOR: applet/stconn: Add a SE flag to specify an endpoint does not expect data

An endpoint should now set SE_FL_EXP_NO_DATA flag if it does not expect any
data from the opposite endpoint. This way, the stream will be able to
disable any read timeout on the opposite endpoint. Applets should use
applet_expect_no_data() and applet_expect_data() functions to set or clear
the flag. For now, only dns and sink forwarder applets are concerned.