afinter: adjusting window size for internal mark mode

[furiel]

Prior this patch, internal mark mode messages were generated independently
from flow control. This means mark mode messages were sent even if the
destination cannot handle messages.

With the patch, mark mode messages can fill now the available window,
as such, internal source can be suspended properly because of them,
and no mark mode messages are generated in that case.

How to test?

Leave syslog-ng running with -Fevd without a destination. You will see that around 100 mark mode messages, the mark mode generation stops. Without this patch, mark mode messages are generated infinitely.

@version: 3.16

options {
  time-reopen(1);
  mark-freq(1);
};

log {
  source { internal(); };
  source { network(port(4444) transport(tcp)); };
  destination { network(localhost port(5555) transport(tcp) mark-mode(internal)); };
  flags(flow-control);
};

Motivated by the discussion of @bazsi @MrAnno @lbudai in
#2129

[bazsi]

looks good to me.

[kira-syslogng]

Build SUCCESS

[MrAnno]

afinter_message_posted() pushes the message into internal_msg_queue.

afinter_source_post() pops the message and calls log_source_post(), where the window size is decremented and ack_needed is set to TRUE.