utils: add a global tmpctx.

I did a brief audit of tmpctx uses, and we do leak them in various
corner cases.  Fortunely, all our daemons are based on some kind of
I/O loop, so it's fairly easy to clean a global tmpctx at that point.

This makes things a bit neater, and slightly more efficient, but also
clearer: I avoided creating a tmpctx in a few places because I didn't
want to add another allocation.  With that penalty removed, I can use
it more freely and hopefully write clearer code.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/utils.h

@@ -16,12 +16,16 @@ char *tal_hex(const tal_t *ctx, const tal_t *data);
 /* Allocate and fill a buffer with the data of this hex string. */
 u8 *tal_hexdata(const tal_t *ctx, const void *str, size_t len);
 
-/* Get a temporary context for this function scope (tal_free at end) */
-tal_t *tal_tmpctx_(const tal_t *ctx, const char *file, unsigned int line);
-#define tal_tmpctx(ctx)							\
-	tal_tmpctx_((ctx), __FILE__, __LINE__)
+/* FIXME: Remove in favor of global */
+#define tal_tmpctx(ctx) tal((ctx), char)
 
-/* Return non-NULL if any tmpctx still allocated. */
-const char *tmpctx_any(void);
+/* Global temporary convenience context: freed in io loop core. */
+extern const tal_t *tmpctx;
+
+/* Initial creation of tmpctx. */
+void setup_tmpctx(void);
+
+/* Free any children of tmpctx. */
+void clean_tmpctx(void);
 
 #endif /* LIGHTNING_COMMON_UTILS_H */