stat: do not rely on undefined behavior in printf formats

stat: do not rely on undefined behavior in printf formats

* src/stat.c (digits, printf_flags): New static vars.
(make_format): New function.
(out_string, out_int, out_uint, out_uint_o, out_uint_x):
(out_minus_zero): Use it to avoid undefined behavior when invoking
printf.
(print_it): Check for invalid conversion specifications such as
%..X and %1-X, which would otherwise rely on undefined behavior
when invoking printf.
* tests/misc/stat-nanoseconds: Check that the "I" printf flag
doesn't mess up in the C locale, as it formerly did on non-GNU
hosts.