MEDIUM: mworker: replace systemd mode by master worker mode

This commit remove the -Ds systemd mode in HAProxy in order to replace
it by a more generic master worker system. It aims to replace entirely
the systemd wrapper in the near future.

The master worker mode implements a new way of managing HAProxy
processes. The master is in charge of parsing the configuration
file and is responsible for spawning child processes.

The master worker mode can be invoked by using the -W flag.  It can be
used either in background mode (-D) or foreground mode. When used in
background mode, the master will fork to daemonize.

In master worker background mode, chroot, setuid and setgid are done in
each child rather than in the master process, because the master process
will still need access to filesystem to reload the configuration.
diff --git a/include/types/global.h b/include/types/global.h
index aeb82ea..ee8e95e 100644
--- a/include/types/global.h
+++ b/include/types/global.h
@@ -44,7 +44,7 @@
 #define	MODE_VERBOSE	0x10
 #define	MODE_STARTING	0x20
 #define	MODE_FOREGROUND	0x40
-#define	MODE_SYSTEMD	0x80
+#define	MODE_MWORKER	0x80    /* Master Worker */
 
 /* list of last checks to perform, depending on config options */
 #define LSTCHK_CAP_BIND	0x00000001	/* check that we can bind to any port */