MEDIUM: New cli option -Ds for systemd compatibility

This patch adds a new option "-Ds" which is exactly like "-D", but instead of
forking n times to get n jobs running and then exiting, prefers to wait for all the
children it just created. With this done, haproxy becomes more systemd-compliant,
without changing anything for other systems.

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
diff --git a/doc/haproxy.1 b/doc/haproxy.1
index 001de15..48717ad 100644
--- a/doc/haproxy.1
+++ b/doc/haproxy.1
@@ -57,6 +57,10 @@
 Start in daemon mode.
 
 .TP
+\fB\-Ds\fP
+Start in systemd daemon mode, keeping a process in foreground.
+
+.TP
 \fB\-q\fP
 Disable messages on output.