fix: main: add check for --debug at the start of main()

The arg_debug flag is set after some function check for it.
A function in this situation is check_kernel_procs().

src/firejail/main.c

@@ -1110,6 +1110,13 @@ int main(int argc, char **argv, char **envp) {
 	if (check_arg(argc, argv, "--quiet", 1) || (env_quiet && strcmp(env_quiet, "yes") == 0))
 		arg_quiet = 1;
 
+	// process --debug
+	const char *env_debug = env_get("FIREJAIL_DEBUG");
+	if (check_arg(argc, argv, "--debug", 1) || (env_debug && strcmp(env_debug, "yes") == 0)) {
+		arg_debug = 1;
+		arg_quiet = 0;
+	}
+
 	// check if the user is allowed to use firejail
 	init_cfg(argc, argv);