Sender code consolidation, on/off commands for the dccp sender.

Sender code consolidation, on/off commands for the dccp sender.

This patch moves a bunch of code from the http sender to
send_common.c to make it available also for the dccp sender.

To this aim a new structure sender_status is introduced which
holds the configuration and the current status of the http/dccp
sender. Most of the new functions in send_common.c take a pointer
to such a structure.

This allows to implement the off/on/allow/deny/info commands
as a one-liner for both the dccp and the http sender.

The new server config option dccp_max_clients allows to restrict the
number of simultaneous connections to the dccp sender.