feature(cp): implement archive & -L

[sylvestre]

Real life test case:

set -e

rm -rf dir dir-copy-archive-rust
mkdir dir
echo "a" > dir/1
echo "b" > dir/2
touch -d "2 hours ago" dir/1
chmod 705 dir/*
cd dir
ln -s 1 1.link
ln -s 2 2.link
cd -
cp --archive dir dir-copy-archive
ls -al dir-copy-archive|grep "rwx---r-x"
ls -al dir-copy-archive|grep "1.link -> 1"
./target/debug/coreutils cp --archive -v dir dir-copy-archive-rust
ls -al dir-copy-archive-rust|grep "rwx---r-x"
ls -al dir-copy-archive-rust|grep "1.link -> 1"
ls -al dir-copy-archive-rust|grep "2.link -> 2"

[sylvestre]

Made progress, I am ignoring the error when it is a symlink.

[sylvestre]

I am planning to land this Thursday 5st if no objection!

[rivy]

I haven't really had the time to review this... (busy with COVID uptick at the hospitals).
There are a few style notes/warnings and several CodeCov comments about new, none-test-covered, code.
And I haven't had the chance to actually run/test the new option.
I should be able to get to it over next weekend if that's not too long.

[rivy]

Minor syntax change request, but I'm concerned that the tests are all "linux" gated.

[rivy]

So, all the tests target only "linux"? ☹️

We definitely need to be testing all platforms. If there is some difficulty in getting them to work on MacOS/Windows, I can offer some time to help at a later date.

And, re-reading the docs, I think target_os = "unix" is incorrect; it should be target_os = "linux".

"unix" is a target_family.

[sylvestre]

Yeah, I don't know how to fix these :/

Symlinks are tricky on Windows

[sylvestre]

Sorry, I know this isn't perfect but this works and it is improving the overall code coverage.
Landing as it has been around since August