unix: disable relaxed accept() by default

Don't use the relaxed accept() algorithm introduced in be2a217 unless
explicitly requested. It causes a 50+% performance drop on some node.js
benchmarks:

  $ alias bench='out/Release/node benchmark/http_simple_auto.js \
    -c 10 -n 50000 bytes/1 2>&1 | grep Req'

  $ UV_TCP_SINGLE_ACCEPT=0 bench
  Requests per second:    12331.84 [#/sec] (mean)

  $ UV_TCP_SINGLE_ACCEPT=1 bench
  Requests per second:    3944.63 [#/sec] (mean)

src/unix/tcp.c

@@ -233,7 +233,7 @@ int uv_tcp_listen(uv_tcp_t* tcp, int backlog, uv_connection_cb cb) {
 
   if (single_accept == -1) {
     const char* val = getenv("UV_TCP_SINGLE_ACCEPT");
-    single_accept = (val == NULL) || (atoi(val) != 0); /* on by default */
+    single_accept = (val != NULL && atoi(val) != 0);  /* Off by default. */
   }
 
   if (single_accept)