Moved credential-related code to nxt_credential.c.

This is required to avoid include cycles, as some nxt_clone_* functions
depend on the credential structures, but nxt_process depends on clone
structures.

src/nxt_runtime.c

@@ -705,7 +705,7 @@ nxt_runtime_conf_init(nxt_task_t *task, nxt_runtime_t *rt)
     }
 
     if (rt->capabilities.setid) {
-        ret = nxt_user_cred_get(task, rt->mem_pool, &rt->user_cred,
+        ret = nxt_credential_get(task, rt->mem_pool, &rt->user_cred,
                                 rt->group);
 
         if (nxt_slow_path(ret != NXT_OK)) {