Implement statistics display in checkpoint actions.

* NEWS: Update.
* configure.ac: Version 1.27.90
* gnulib.modules: Add fprintftime.
* doc/tar.texi: Document the "totals" action and new format specifiers
for echo and ttyout checkpoint actions.
* src/buffer.c (compute_duration): Return computed value.
(print_stats): Don't print trailing newline.  Return number of
characters output.
(format_total_stats): New function.
(print_total_stats): Rewrite via format_total_stats.
* src/checkpoint.c (checkpoint_opcode) <cop_totals>: New opcode.
(checkpoint_compile_action): Handle cop_totals.
(expand_checkpoint_string): Remove.
(format_checkpoint_string): New function to be used instead of
expand_checkpoint_string.  All callers updated.
* src/common.h (TF_READ,TF_WRITE)
(TF_DELETED): New constants.
(format_total_stats,print_total_stats): New protos.