MINOR: trace: accept trace spec right after "-dt" on the command line

I continue to mistakenly set the traces using "-dtXXX" and to have to
refer to the doc to figure that it requires a separate argument and
differs from some other options. Worse, "-dthelp" doesn't say anything
and silently ignores the argument.

Let's make the parser take whatever follows "-dt" as the argument if
present, otherwise take the next one (as it currently does). Doing
this even allows to simplify the code, and is easier to figure the
syntax since "-dthelp" now works.

Author: wtarreau

src/haproxy.c

@@ -697,7 +697,7 @@ static void usage(char *name)
 		"        -vq/-vqs/-vqb only displays version, short version, branch.\n"
 		"        -d enters debug mode ; -db only disables background mode.\n"
 		"        -dM[<byte>,help,...] debug memory (default: poison with <byte>/0x50)\n"
-		"        -dt activate traces on stderr\n"
+		"        -dt activate traces on stderr; see '-dt help'\n"
 		"        -V enters verbose mode (disables quiet mode)\n"
 		"        -D goes daemon ; -C changes to <dir> before loading files.\n"
 		"        -W master-worker mode.\n"
@@ -1624,24 +1624,26 @@ static void init_args(int argc, char **argv)
 				kwd_dump = flag + 2;
 			}
 			else if (*flag == 'd' && flag[1] == 't') {
-				if (argc > 1 && argv[1][0] != '-') {
-					int ret = trace_parse_cmd(argv[1], &err_msg);
-					if (ret <= -1) {
-						if (ret < -1) {
-							ha_alert("-dt: %s.\n", err_msg);
-							ha_free(&err_msg);
-							exit(EXIT_FAILURE);
-						}
-						else {
-							printf("%s\n", err_msg);
-							ha_free(&err_msg);
-							exit(0);
-						}
-					}
+				char *arg = flag + 2;
+				int ret;
+
+				if (!*arg && argc > 1 && argv[1][0] != '-') {
+					arg = argv[1];
 					argc--; argv++;
 				}
-				else {
-					trace_parse_cmd(NULL, NULL);
+
+				ret = trace_parse_cmd(arg, &err_msg);
+				if (ret <= -1) {
+					if (ret < -1) {
+						ha_alert("-dt: %s.\n", err_msg);
+						ha_free(&err_msg);
+						exit(EXIT_FAILURE);
+					}
+					else {
+						printf("%s\n", err_msg);
+						ha_free(&err_msg);
+						exit(0);
+					}
 				}
 			}
 #ifdef HA_USE_KTLS

src/trace.c

@@ -1008,7 +1008,7 @@ int trace_parse_cmd(const char *arg_src, char **errmsg)
 	char *arg, *oarg;
 	char *saveptr;
 
-	if (!arg_src) {
+	if (!arg_src || !*arg_src) {
 		/* No trace specification, activate all sources on error level. */
 		struct trace_source *src = NULL;