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-en.txt b/doc/haproxy-en.txt
index 3d2a2be..4c673d3 100644
--- a/doc/haproxy-en.txt
+++ b/doc/haproxy-en.txt
@@ -51,6 +51,7 @@
        = 'maxconn' in 'listen' or 'default' sections
     -d starts in foregreound with debugging mode enabled
     -D starts in daemon mode
+    -Ds starts in systemd daemon mode
     -q disable messages on output
     -V displays messages on output even when -q or 'quiet' are specified.
     -c only checks config file and exits with code 0 if no error was found, or
diff --git a/doc/haproxy-fr.txt b/doc/haproxy-fr.txt
index 87bb972..0e36724 100644
--- a/doc/haproxy-fr.txt
+++ b/doc/haproxy-fr.txt
@@ -53,6 +53,7 @@
        = 'maxconn' dans les sections 'listen' ou 'default'
     -d active le mode debug
     -D passe en daemon
+    -Ds passe en daemon systemd
     -q désactive l'affichage de messages sur la sortie standard.
     -V affiche les messages sur la sortie standard, même si -q ou 'quiet' sont
        spécifiés.
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.