Add support for removing directories.

It's really only supported in Ext2FS since SynthFS doesn't really want you
mucking around with its files. This is pretty neat though :^)

I ran into some trouble with HashMap while working on this but opted to work
around it and leave that for a separate investigation.

Userland/rmdir.cpp

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