Update MentOs code to the latest development version.

programs/cat.c

@@ -0,0 +1,40 @@
+///                MentOS, The Mentoring Operating system project
+/// @file cat.c
+/// @brief `cat` program.
+/// @copyright (c) 2014-2021 This file is distributed under the MIT License.
+/// See LICENSE.md for details.
+
+#include <fcntl.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/unistd.h>
+#include <strerror.h>
+
+int main(int argc, char **argv)
+{
+    if (argc != 2) {
+        printf("%s: missing operand.\n", argv[0]);
+        printf("Try '%s --help' for more information.\n\n", argv[0]);
+        return 1;
+    }
+    if (strcmp(argv[1], "--help") == 0) {
+        printf("Prints the content of the given file.\n");
+        printf("Usage:\n");
+        printf("    %s <file>\n\n", argv[0]);
+        return 0;
+    }
+    int fd = open(argv[1], O_RDONLY, 42);
+    if (fd < 0) {
+        printf("%s: %s: %s\n\n", argv[0], strerror(errno), argv[1]);
+        return 1;
+    }
+    char buffer[BUFSIZ];
+    // Put on the standard output the characters.
+    while (read(fd, buffer, BUFSIZ) > 0) {
+        puts(buffer);
+    }
+    putchar('\n');
+    putchar('\n');
+    close(fd);
+    return 0;
+}