unix,win: look up UV_THREADPOOL_SIZE with uv_os_getenv (libuv#4888)

Use libuv's OS-independent implementation to look up UV_THREADPOOL_SIZE.

Fixes: libuv#4887

Author: konakona418

src/threadpool.c

@@ -194,11 +194,21 @@ void uv__threadpool_cleanup(void) {
 static void init_threads(void) {
   uv_thread_options_t config;
   unsigned int i;
+  size_t buflen;
+  char buf[16];
   const char* val;
+  int err;
+
   uv_sem_t sem;
 
   nthreads = ARRAY_SIZE(default_threads);
-  val = getenv("UV_THREADPOOL_SIZE");
+
+  buflen = ARRAY_SIZE(buf);
+  err = uv_os_getenv("UV_THREADPOOL_SIZE", buf, &buflen);
+  val = NULL;
+  if (err == 0)
+    val = buf;
+  
   if (val != NULL)
     nthreads = atoi(val);
   if (nthreads == 0)