numfmt: avoid integer overflow when rounding

numfmt: avoid integer overflow when rounding

Due to existing limits this is usually triggered
with an increased precision.  We also add further
restrictions to the output of increased precision numbers.

* src/numfmt.c (simple_round): Avoid intmax_t overflow.
(simple_strtod_int): Count digits consistently
for precision loss and overflow detection.
(prepare_padded_number): Include the precision
when excluding numbers to output, since the precision
determines the ultimate values used in the rounding scheme
in double_to_human().
* tests/misc/numfmt.pl: Add previously failing test cases.
* NEWS: Mention the fix.