Bigger iovec buffer in ngx_readv_chain().

This helps to reduce likelyhood of memory allocations in ngx_readv_chain(),
which are known to lead to noticeable effects in some cases, see
http://mailman.nginx.org/pipermail/nginx/2014-July/044512.html.
diff --git a/src/os/unix/ngx_readv_chain.c b/src/os/unix/ngx_readv_chain.c
index 8836c81..e4eb5ff 100644
--- a/src/os/unix/ngx_readv_chain.c
+++ b/src/os/unix/ngx_readv_chain.c
@@ -10,7 +10,11 @@
 #include <ngx_event.h>
 
 
-#define NGX_IOVS  16
+#if (IOV_MAX > 64)
+#define NGX_IOVS  64
+#else
+#define NGX_IOVS  IOV_MAX
+#endif
 
 
 #if (NGX_HAVE_KQUEUE)