This commit replaces generic Results with UResults in some key
functions in numfmt. As a result of this, we can provide different
exit codes for different errors, which resolves ~70 failing test
cases in the GNU numfmt.pl test suite.
Fails from time to time with
```
---- test_numfmt::test_should_calculate_implicit_padding_per_free_argument stdout ----
current_directory_resolved:
run: /target/x86_64-unknown-linux-musl/debug/coreutils numfmt --from=auto 1Ki 2K
thread 'test_numfmt::test_should_calculate_implicit_padding_per_free_argument' panicked at 'failed to write to stdin of child: Broken pipe (os error 32)', tests/common/util.rs:859:21
```
When converting to SI or IEC, produce values that align with the conventions
used by GNU numfmt.
- values > 10 are represented without a decimal place, so 10000 becomes 10K
instead of 10.0K
- when truncating, take the ceiling of the value, so 100001 becomes 101K
- values < 10 are truncated to the highest tenth, so 1001 becomes 1.1K
closes#1726
Adjust header option handling to prohibit passing a value of 0 to align
with GNU numfmt. Also report header option parse errors as GNU does.
closes#1708
Align with GNU numfmt by trimming leading whitespace from supplied values.
If the user did not specify a padding, calculate an implied padding from
the leading whitespace and the value.
Also track closer to GNU numfmt’s error message format.
