Finally hook up the mkdir code to a syscall.

Added a /bin/mkdir that makes directories. How very neat :^)
There are various limitations because of missing functionality.

Userland/mkdir.cpp

@@ -0,0 +1,21 @@
+#include <stdio.h>
+#include <unistd.h>
+#include <errno.h>
+#include <string.h>
+#include <stdlib.h>
+#include <assert.h>
+#include <sys/stat.h>
+
+int main(int argc, char** argv)
+{
+    if (argc != 2) {
+        printf("usage: mkdir <path>\n");
+        return 1;
+    }
+    int rc = mkdir(argv[1], 0755);
+    if (rc < 0) {
+        perror("mkdir");
+        return 1;
+    }
+    return 0;
+}