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/x509/cert_app.c b/programs/x509/cert_app.c
index dd5c508..e555bdb 100644
--- a/programs/x509/cert_app.c
+++ b/programs/x509/cert_app.c
@@ -475,7 +475,7 @@
 
 exit:
 
-    mbedtls_net_close( &server_fd );
+    mbedtls_net_free( &server_fd );
     mbedtls_x509_crt_free( &cacert );
     mbedtls_x509_crt_free( &clicert );
 #if defined(MBEDTLS_X509_CRL_PARSE_C)