BUG/MINOR: systemd: potential zombie processes

In systemd mode (-Ds), the master haproxy process is waiting for each
child to exit in a specific order. If a process die when it's not his
turn, it will become a zombie process until every processes exit.

The master is now waiting for any process to exit in any order.

This patch should be backported to 1.7, 1.6 and 1.5.

src/haproxy.c

@@ -1898,7 +1898,7 @@ int main(int argc, char **argv)
 				/* it's OK because "-Ds -f x" is the shortest form going here */
 				memcpy(argv[0] + strlen(argv[0]), "-master", 8);
 				for (proc = 0; proc < global.nbproc; proc++)
-					while (waitpid(children[proc], NULL, 0) == -1 && errno == EINTR);
+					while (waitpid(-1, NULL, 0) == -1 && errno == EINTR);
 			}
 			exit(0); /* parent must leave */
 		}