Add /dev/{stdin,stdout,stderr} as symlinks to /proc/self/fd/{0,1,2}

Also change /bin/cat to open /dev/stdin if no arguments are provided.

Kernel/sync.sh

@@ -13,6 +13,9 @@ mknod mnt/dev/psaux c 10 1
 mknod mnt/dev/ptmx c 5 2
 mkdir mnt/dev/pts
 mknod mnt/dev/gui_events c 66 1
+ln -s /proc/self/fd/0 mnt/dev/stdin
+ln -s /proc/self/fd/1 mnt/dev/stdout
+ln -s /proc/self/fd/2 mnt/dev/stderr
 cp -vR ../Base/* mnt/
 chown -vR 100:100 mnt/users/anon
 cp -v ../Userland/sh mnt/bin/sh

Userland/cat.cpp

@@ -8,13 +8,10 @@
 
 int main(int argc, char** argv)
 {
-    if (argc != 2) {
-        printf("usage: cat <file>\n");
-        return 1;
-    }
-    int fd = open(argv[1], O_RDONLY);
+    const char* input_file = argc > 1 ? argv[1] : "/dev/stdin";
+    int fd = open(input_file, O_RDONLY);
     if (fd == -1) {
-        printf("failed to open %s: %s\n", argv[1], strerror(errno));
+        printf("failed to open %s: %s\n", input_file, strerror(errno));
         return 1;
     }
     for (;;) {