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.

stubs/no-ldisc.c

@@ -0,0 +1,37 @@
+#include "putty.h"
+
+struct Ldisc_tag {
+    int dummy;
+};
+
+Ldisc *ldisc_create(Conf *conf, Terminal *term, Backend *backend, Seat *seat)
+{
+    Ldisc *ldisc = snew(Ldisc);
+    memset(ldisc, 0, sizeof(Ldisc));
+    return ldisc;
+}
+
+void ldisc_configure(Ldisc *ldisc, Conf *conf)
+{
+}
+
+void ldisc_free(Ldisc *ldisc)
+{
+    sfree(ldisc);
+}
+
+void ldisc_echoedit_update(Ldisc *ldisc)
+{
+}
+
+void ldisc_provide_userpass_le(Ldisc *ldisc, TermLineEditor *le)
+{
+}
+
+void ldisc_check_sendok(Ldisc *ldisc)
+{
+}
+
+void ldisc_send(Ldisc *ldisc, const void *vbuf, int len, bool interactive)
+{
+}