MINOR: http-rules: Make set-header and add-header custom actions

MINOR: http-rules: Make set-header and add-header custom actions

Now, these actions use their own dedicated function and are no longer handled
"in place" during the HTTP rules evaluation. Thus the action names
ACT_HTTP_SET_HDR and ACT_HTTP_ADD_VAL are removed. The action type is now set to
0 to set a header (so remove existing ones if any and add a new one) or to 1 to
add a header (add without remove).