Kernel: Implement basic module unloading :^)

Kernel modules can now be unloaded via a syscall. They get a chance to
run some code of course. Before deallocating them, we call their
"module_fini" symbol.

Userland/modunload.cpp

@@ -0,0 +1,15 @@
+#include <serenity.h>
+#include <string.h>
+
+int main(int argc, char** argv)
+{
+    (void)argc;
+    (void)argv;
+    const char* name = "FIXME";
+    int rc = module_unload(name, strlen(name));
+    if (rc < 0) {
+        perror("module_unload");
+        return 1;
+    }
+    return 0;
+}