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

programs/ssl/ssl_mail_client.c

@@ -819,7 +819,7 @@ int main( int argc, char *argv[] )
 
 exit:
 
-    mbedtls_net_close( &server_fd );
+    mbedtls_net_free( &server_fd );
     mbedtls_x509_crt_free( &clicert );
     mbedtls_x509_crt_free( &cacert );
     mbedtls_pk_free( &pkey );