Reorganise the stubs collection.

I made a specific subdirectory 'stubs' to keep all the link-time stub
modules in, like notiming.c. And I put _one_ run-time stub in it,
namely nullplug.c. But the rest of the runtime stubs went into utils.

I think it's better to keep all the stubs together, so I've moved all
the null*.c in utils into stubs (with the exception of nullstrcmp.c,
which means the 'null' in a different sense). Also, fiddled with the
naming to be a bit more consistent, and stated in the new CMakeLists
the naming policy that distinguishes no-*.c from null-*.c.

stubs/no-rand.c

@@ -0,0 +1,22 @@
+/*
+ * Stub implementations of RNG functions for applications without an RNG. 
+ */
+
+#include "putty.h"
+
+void random_read(void *out, size_t size)
+{
+    unreachable("Random numbers are not available in this application");
+}
+
+void random_save_seed(void)
+{
+}
+
+void random_destroy_seed(void)
+{
+}
+
+void noise_ultralight(NoiseSourceId id, unsigned long data)
+{
+}