control: refactor to separate control operations from control channel

Earlier the control subsystem tightly coupled the implementation of the
control channel using UNIX domain sockets and the responses syslog-ng was
performing for each control request.

This patch nicely separates these concepts in both the server and the client
side of the control channel.

Signed-off-by: Juhasz Viktor <jviktor@balabit.hu>
Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>