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