Var: dynamic variables support.

This commit adds the variables $arg_NAME, $header_NAME, and $cookie_NAME.

src/nxt_conf.h

@@ -72,6 +72,7 @@ typedef struct {
     nxt_mp_t             *pool;
     nxt_str_t            error;
     void                 *ctx;
+    nxt_array_t          *var_fields;  /* of nxt_var_field_t */
     nxt_mp_t             *conf_pool;
     nxt_uint_t           ver;
 } nxt_conf_validation_t;