Generate graph from the traffic observable by busctl monitor.

Generate graph from the traffic observable by busctl monitor.

Busctl dot is used to visualize IPC traffic.
It generate dot output whitch can be visualized e.g. by using graphviz.
Also adds filtering by sender/receiver pid.
Internally busctl dot works similar to busctl capture.
It reuses monitor() framework.

--pid=PID -only show messages where sender or receiver pid is equal to PID
--sender-pid=PID -only show messages where sender pid is equal to PID
--receiver-pid=PID -same as above, but filtering by receiver pid.

Signed-off-by: Dawid Kuczma <d.kuczma@partner.samsung.com>
Change-Id: Id9ce46f5085dde10010e89057f78d58b3088d3bc