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-timing.c

@@ -0,0 +1,21 @@
+/*
+ * no-timing.c: stub version of timing API.
+ *
+ * Used in any tool which needs a subsystem linked against the
+ * timing API but doesn't want to actually provide timing. For
+ * example, key generation tools need the random number generator,
+ * but they don't want the hassle of calling noise_regular() at
+ * regular intervals - and they don't _need_ it either, since they
+ * have their own rigorous and different means of noise collection.
+ */
+
+#include "putty.h"
+
+unsigned long schedule_timer(int ticks, timer_fn_t fn, void *ctx)
+{
+    return 0;
+}
+
+void expire_timer_context(void *ctx)
+{
+}