checkpoint actions: further improvements.

* NEWS: Update.
* doc/tar.texi: Update.
* src/buffer.c (print_stats): Avoid use of additional string buffer.
Allow for text to be NULL.
Call gettext if it is not.
(format_total_stats): Don't use gettext when calling print_stats.
* src/checkpoint.c (def_format): Change default format.
(format_checkpoint_string): Implement optional arguments for
T conversion.
(finish_checkpoint_actions): Rename to checkpoint_flush_actions,
make extern.  All uses changed.
* src/common.h (checkpoint_flush_actions): New proto.
* src/tar.c (main): Set error_hook