common/utils.h: add tal_arr_expand helper.

We do this a lot, and had boutique helpers in various places.  So add
a more generic one; for convenience it returns a pointer to the new
end element.

I prefer the name tal_arr_expand to tal_arr_append, since it's up to
the caller to populate the new array entry.

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

common/utils.h

@@ -19,6 +19,16 @@ 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);
 
+/* Helper macro to extend tal_arr and return pointer new last element. */
+#if HAVE_STATEMENT_EXPR
+/* More efficient version calls tal_count() once */
+#define tal_arr_expand(p)					\
+	({ size_t n = tal_count(*p); tal_resize((p), n+1); *(p) + n; })
+#else
+#define tal_arr_expand(p) \
+	(tal_resize((p), tal_count(*(p))+1), (*p) + tal_count(*(p))-1)
+#endif
+
 /* Use the POSIX C locale. */
 void setup_locale(void);