Router: refactored variable pass.

Since the "pass" option supports both strings and variables, a generic
nxt_var_t structure can be used in the configuration phase, and the "name"
field in actions is redundant.

No functional changes.

src/nxt_var.h

@@ -30,6 +30,9 @@ nxt_is_var(nxt_str_t *str)
 }
 
 
+void nxt_var_raw(nxt_var_t *var, nxt_str_t *str);
+nxt_bool_t nxt_var_is_const(nxt_var_t *var);
+
 nxt_int_t nxt_var_register(nxt_var_decl_t *decl, size_t n);
 nxt_int_t nxt_var_index_init(void);
 nxt_var_t *nxt_var_compile(nxt_str_t *str, nxt_mp_t *mp);