Rename mbedtls_net_close() to mbedtls_net_free()

close() may be more meaningful, but free() is symmetric with _init(), and more
consistent with all other modules
diff --git a/programs/ssl/dtls_server.c b/programs/ssl/dtls_server.c
index e1da0f7..a3839c6 100644
--- a/programs/ssl/dtls_server.c
+++ b/programs/ssl/dtls_server.c
@@ -256,7 +256,7 @@
     }
 #endif
 
-    mbedtls_net_close( &client_fd );
+    mbedtls_net_free( &client_fd );
 
     mbedtls_ssl_session_reset( &ssl );
 
@@ -401,8 +401,8 @@
     }
 #endif
 
-    mbedtls_net_close( &client_fd );
-    mbedtls_net_close( &listen_fd );
+    mbedtls_net_free( &client_fd );
+    mbedtls_net_free( &listen_fd );
 
     mbedtls_x509_crt_free( &srvcert );
     mbedtls_pk_free( &pkey );