BUG/MINOR: systemd: make the wrapper return a non-null status code on error

When execv() fails to execute the haproxy executable, it's important to
return an error instead of pretending everything is cool. This fix should
be backported to 1.6 and 1.5 in order to improve the overall reliability
under systemd.
(cherry picked from commit 7643d09dca4d0eed97ba3c29d4f4fd1f037f96ae)
(cherry picked from commit 03c706d71e2d314670d2ebb4dfa48fd3b793b361)
diff --git a/src/haproxy-systemd-wrapper.c b/src/haproxy-systemd-wrapper.c
index 4e4d039..f9e2e0c 100644
--- a/src/haproxy-systemd-wrapper.c
+++ b/src/haproxy-systemd-wrapper.c
@@ -94,7 +94,7 @@
 		fprintf(stderr, "\n");
 
 		execv(argv[0], argv);
-		exit(0);
+		exit(1);
 	}
 }