Don't exit mbedtls_net_poll on interruption of select

If the select UNIX system call is interrupted by a signal handler,
it is not automatically restarted but returns EINTR. This commit
modifies the use of select in mbedtls_net_poll from net_sockets.c
to retry the select call in this case.
diff --git a/library/net_sockets.c b/library/net_sockets.c
index 2d1c108..e63e496 100644
--- a/library/net_sockets.c
+++ b/library/net_sockets.c
@@ -470,8 +470,11 @@
     tv.tv_sec  = timeout / 1000;
     tv.tv_usec = ( timeout % 1000 ) * 1000;
 
-    ret = select( fd + 1, &read_fds, &write_fds, NULL,
-                  timeout == (uint32_t) -1 ? NULL : &tv );
+    do
+    {
+        ret = select( fd + 1, &read_fds, &write_fds, NULL,
+                      timeout == (uint32_t) -1 ? NULL : &tv );
+    } while( ret == EINTR );
 
     if( ret < 0 )
         return( MBEDTLS_ERR_NET_POLL_FAILED );