Add lots more stub versions of standard code modules.

These are all going to be used by a test program I have in the works,
which will need to link against a lot more of the code base than any
so far. So we need a pile of new stubs.

The trickiest of these was stubs/no-network.c, which had to
conditionally define a couple of extra network functions, because
there are Windows-specific plug_closing_system_error and
plug_closing_winsock_error functions.

unix/stubs/no-uxsel.c

@@ -0,0 +1,31 @@
+/*
+ * Stub version of uxsel.c, for test programs.
+ */
+
+#include "putty.h"
+
+void uxsel_init(void)
+{
+}
+
+void uxsel_set(int fd, int rwx, uxsel_callback_fn callback)
+{
+}
+
+void uxsel_del(int fd)
+{
+}
+
+int next_fd(int *state, int *rwx)
+{
+    return -1;
+}
+
+int first_fd(int *state, int *rwx)
+{
+    return -1;
+}
+
+void select_result(int fd, int event)
+{
+}