Most of these changes were sourced from mv's existing backup control
implementation. A later commit will update the mv utility to use this
new share backup control.
Fails trom time to time with:
```
info: installing component 'rustc'
memory allocation of 16777216 bytes failed
Error: The process 'C:\Rust\.cargo\bin\rustup.exe' failed with exit code 3221226505
```
on Build (windows-latest, i686-pc-windows-gnu, feat_os_windows)
Add some abstractions to simplify the `rbuf_but_last_n_lines()`
function, which implements the "take all but the last `n` lines"
functionality of the `head` program. This commit adds
- `RingBuffer`, a fixed-size ring buffer,
- `ZLines`, an iterator over zero-terminated "lines",
- `TakeAllBut`, an iterator over all but the last `n` elements of an
iterator.
These three together make the implementation of
`rbuf_but_last_n_lines()` concise.
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
```
`sort` supports three ways to specify the sort mode: a long option
(e.g. --numeric-sort), a short option (e.g. -n) and the sort flag
(e.g. --sort=numeric).
This adds support for the sort flag.
Additionally, sort modes now conflict, which means that an error is
shown when multiple modes are passed, instead of silently picking a mode.
For consistency, I added the `random` sort mode to the `SortMode` enum,
instead of it being a bool flag.
Change the behavior of `wc` to print the counts for a file as soon as
it is computed, instead of waiting to compute the counts for all files
before writing any output to `stdout`. The new behavior matches the
behavior of GNU `wc`.
The old behavior looked like this (the word "hello" is entered on
`stdin`):
$ wc emptyfile.txt -
hello
0 0 0 emptyfile.txt
1 1 6
1 1 6 total
The new behavior looks like this:
$ wc emptyfile.txt -
0 0 0 emptyfile.txt
hello
1 1 6
1 1 6 total
