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-cmdline.c

@@ -0,0 +1,22 @@
+/*
+ * no-cmdline.c - stubs in applications which don't do the
+ * standard(ish) PuTTY tools' command-line parsing
+ */
+
+#include <stdio.h>
+#include <assert.h>
+#include <stdlib.h>
+#include "putty.h"
+
+/*
+ * Stub version of the function in cmdline.c which provides the
+ * password to SSH authentication by remembering it having been passed
+ * as a command-line option. If we're not doing normal command-line
+ * handling, then there is no such option, so that function always
+ * returns failure.
+ */
+SeatPromptResult cmdline_get_passwd_input(
+    prompts_t *p, cmdline_get_passwd_input_state *state, bool restartable)
+{
+    return SPR_INCOMPLETE;
+}