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

library/net.c

@@ -521,7 +521,7 @@ int mbedtls_net_send( void *ctx, const unsigned char *buf, size_t len )
 /*
  * Gracefully close the connection
  */
-void mbedtls_net_close( mbedtls_net_context *ctx )
+void mbedtls_net_free( mbedtls_net_context *ctx )
 {
     if( ctx->fd == -1 )
         return;