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-print.c

@@ -0,0 +1,38 @@
+/*
+ * Stub implementation of the printing interface for PuTTY, for the
+ * benefit of non-printing terminal applications.
+ */
+
+#include <assert.h>
+#include <stdio.h>
+#include "putty.h"
+
+struct printer_job_tag {
+    int dummy;
+};
+
+printer_job *printer_start_job(char *printer)
+{
+    return NULL;
+}
+
+void printer_job_data(printer_job *pj, const void *data, size_t len)
+{
+}
+
+void printer_finish_job(printer_job *pj)
+{
+}
+
+printer_enum *printer_start_enum(int *nprinters_ptr)
+{
+    *nprinters_ptr = 0;
+    return NULL;
+}
+char *printer_get_name(printer_enum *pe, int i)
+{
+    return NULL;
+}
+void printer_finish_enum(printer_enum *pe)
+{
+}