Backport STR_STATIC_INIT and STR_NULL macros from trunk, to simplify

backporting changes in modules.

str.h

@@ -37,5 +37,7 @@ struct _str{
 
 typedef struct _str str;
 
+#define STR_STATIC_INIT(v) {(v), sizeof(v) - 1}
+#define STR_NULL {NULL, 0}
 
 #endif