MEDIUM: mworker-prog: implement program for master-worker

This patch implements the external binary support in the master worker.

To configure an external process, you need to use the program section,
for example:

	program dataplane-api
		command ./dataplane_api

Those processes are launched at the same time as the workers.

During a reload of HAProxy, those processes are dealing with the same
sequence as a worker:

  - the master is re-executed
  - the master sends a USR1 signal to the program
  - the master launches a new instance of the program

During a stop, or restart, a SIGTERM is sent to the program.
diff --git a/include/proto/mworker.h b/include/proto/mworker.h
index 02a10de..bf8317f 100644
--- a/include/proto/mworker.h
+++ b/include/proto/mworker.h
@@ -33,5 +33,7 @@
 
 int mworker_child_nb();
 
+int mworker_ext_launch_all();
+
 
 #endif /* PROTO_MWORKER_H_ */