add NGX_ENETDOWN, NGX_ENETUNREACH, and NGX_EHOSTDOWN

src/core/ngx_connection.c

diff --git a/src/core/ngx_connection.c b/src/core/ngx_connection.c
index e0a6ba2..b31daaa 100644
--- a/src/core/ngx_connection.c
+++ b/src/core/ngx_connection.c
@@ -767,6 +767,9 @@
         || err == NGX_ENOTCONN
         || err == NGX_ETIMEDOUT
         || err == NGX_ECONNREFUSED
+        || err == NGX_ENETDOWN
+        || err == NGX_ENETUNREACH
+        || err == NGX_EHOSTDOWN
         || err == NGX_EHOSTUNREACH)
     {
         switch (c->log_error) {