Kernel: Add link() syscall to create hard links.

This accidentally grew into a little bit of VFS cleanup as well.

Also add a simple /bin/ln implementation to exercise it.

Userland/ln.cpp

@@ -0,0 +1,18 @@
+#include <stdio.h>
+#include <errno.h>
+#include <unistd.h>
+
+int main(int argc, char** argv)
+{
+    if (argc != 3) {
+        fprintf(stderr, "usage: ln <old-path> <new-path>\n");
+        return 1;
+    }
+    int rc = link(argv[1], argv[2]);
+    if (rc < 0) {
+        perror("link");
+        return 1;
+    }
+    return 0;
+}
+