stat: optimize and translate default format strings

stat: optimize and translate default format strings

Yes, this patch intentionally leaks the results of default_format(),
since it is called only twice, and since the results are in scope
until main() exits.  Not worth the extra code to pacify valgrind.

* src/stat.c (main): Hoist default format computation out of loop.
(do_statfs, do_stat): Move default format generation...
(default_format): ...into new function.  Allocate the result in
pieces, rather than repeating mostly-similar chunks.  Allow
translation of verbose format.  Pass a second format to do_stat,
for the one aspect of the default format that is conditional on
file type.
* NEWS: Document the translation aspect.