Add serial port support

With this patch we add serial port support via -s (--serial) command
line option.

main.c

@@ -4,17 +4,71 @@
  * Copyright (c) 2023 Andrea Cervesato <[email protected]>
  */
 
-#include <unistd.h>
+#include <fcntl.h>
 #include <stdio.h>
+#include <errno.h>
+#include <string.h>
+#include <unistd.h>
+#include <sys/stat.h>
+#include <sys/types.h>
 #include "ltx.h"
 
-int main(void)
+int main(int argc, char *argv[])
 {
+	int stdin_fd = STDIN_FILENO;
+	int stdout_fd = STDOUT_FILENO;;
+
+	if (argc >= 2 && strcmp(argv[1], "-i") && strcmp(argv[1], "--interactive")) {
+		if (!strcmp(argv[1], "-s") || !strcmp(argv[1], "--serial")) {
+			if (argc != 3) {
+				printf("Serial port is not defined\n");
+				return 1;
+			}
+
+			const char *port = argv[2];
+
+			if (access(port, F_OK) != 0) {
+				printf("%s doesn't exist\n", port);
+				return 1;
+			}
+
+			stdin_fd = open(port, O_RDONLY | O_NOCTTY);
+			if (stdin_fd == -1) {
+				printf("Can't open stdin %s (%s)\n", port, strerror(errno));
+				return 1;
+			}
+
+			stdout_fd = open(port, O_WRONLY | O_NOCTTY);
+			if (stdout_fd == -1) {
+				printf("Can't open stdout %s (%s)\n", port, strerror(errno));
+				return 1;
+			}
+		} else if (!strcmp(argv[1], "-h") || !strcmp(argv[1], "--help")) {
+			printf(
+				"Usage: ./ltx [-h|-i|-s]\n\n"
+				"  -i | --interactive  communicate via stdin|stdout (default)\n"
+				"  -s | --serial       communicate via serial port\n"
+				"  -h | --help         print help message\n"
+				"  -v | --version      print version\n\n"
+			);
+			return 0;
+		} else if (!strcmp(argv[1], "-v") || !strcmp(argv[1], "--version")) {
+			printf("%s\n", VERSION);
+			return 0;
+		} else {
+			printf("Unknown parameter: %s\n", argv[1]);
+			return 1;
+		}
+	}
+
 	struct ltx_session *session;
 
-	session = ltx_session_init(STDIN_FILENO, STDOUT_FILENO);
+	session = ltx_session_init(stdin_fd, stdout_fd);
 	ltx_start_event_loop(session);
 	ltx_session_destroy(session);
 
+	close(stdin_fd);
+	close(stdout_fd);
+
 	return 0;
 }