util: rename join() to strjoin()

This is to match strappend() and the other string related functions.

src/shared/conf-parser.c

@@ -90,7 +90,7 @@ int config_item_perf_lookup(
         else {
                 char *key;
 
-                key = join(section, ".", lvalue, NULL);
+                key = strjoin(section, ".", lvalue, NULL);
                 if (!key)
                         return -ENOMEM;