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

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.