New header file 'defs.h'.

This centralises a few things that multiple header files were
previously defining, and were protecting against each other's
redefinition with ifdefs - small things like structs and typedefs. Now
all those things are in a defs.h which is by definition safe to
include _first_ (out of all the codebase-local headers) and only need
to be defined once.

defs.h

@@ -0,0 +1,37 @@
+/*
+ * defs.h: initial definitions for PuTTY.
+ *
+ * The rule about this header file is that it can't depend on any
+ * other header file in this code base. This is where we define
+ * things, as much as we can, that other headers will want to refer
+ * to, such as opaque structure types and their associated typedefs,
+ * or macros that are used by other headers.
+ */
+
+#ifndef PUTTY_DEFS_H
+#define PUTTY_DEFS_H
+
+typedef struct conf_tag Conf;
+typedef struct backend_tag Backend;
+typedef struct terminal_tag Terminal;
+
+typedef struct Filename Filename;
+typedef struct FontSpec FontSpec;
+
+typedef struct bufchain_tag bufchain;
+
+typedef struct strbuf strbuf;
+
+struct RSAKey;
+
+#include <stdint.h>
+typedef uint32_t uint32;
+
+/* Do a compile-time type-check of 'to_check' (without evaluating it),
+ * as a side effect of returning the value 'to_return'. Note that
+ * although this macro double-*expands* to_return, it always
+ * *evaluates* exactly one copy of it, so it's side-effect safe. */
+#define TYPECHECK(to_check, to_return)                  \
+    (sizeof(to_check) ? (to_return) : (to_return))
+
+#endif /* PUTTY_DEFS_H */