messages of killed clients don't get interrupted

[sthibaul]

Steps to reproduce

• make orca produce long message
• kill it -KILL

Obtained behavior

• the message continues

Expected behavior

• we'd want the message to stop, so that e.g. orca -r restarts speech immediately

Behavior information

• we however don't want the message of spd-say without -w to be stopped

[sthibaul]

We could add a QUIT command that clients use when they are leaving (i.e. spd_close), so that speech dispatcher knows they are exiting properly, and did take care of canceling the message if they wanted to.

If speech dispatcher notices a client sockets gets closed without a QUIT command, it should then cancel its messages, since it probably means the client just crashed.