BUG/MEDIUM: fix segfault when no argument to -x option

This patch fixes a segfault in the command line parser.

When haproxy is launched with -x with no argument and -x is the latest
option in argv it segfaults.

Use usage() insteads of exit() on error.
diff --git a/src/haproxy.c b/src/haproxy.c
index d64058d..2fd387f 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -1279,9 +1279,9 @@
 			else if (*flag == 'q')
 				arg_mode |= MODE_QUIET;
 			else if (*flag == 'x') {
-				if (argv[1][0] == '-') {
-					Alert("Unix socket path expected with the -x flag\n");
-					exit(1);
+				if (argc <= 1 || argv[1][0] == '-') {
+					Alert("Unix socket path expected with the -x flag\n\n");
+					usage(progname);
 				}
 				old_unixsocket = argv[1];
 				argv++;