sort: delete temporary files when sort is terminated

When hitting Ctrl+C sort now deletes any temporary files. To make this easier I
created a new struct `TmpDirWrapper` that handles the creation of new temporary
files and the registration of the signal handler.

chcon: added implementation and integration tests

basenc: add utility

chown: fix unit test

The ToDo list was updated to mark `chcon` as done.

Building and testing `chcon` requires enabling the `feat_selinux` feature. If `make` is used for building, then please specify `SELINUX_ENABLED=1` if building and testing on a system where SELinux is not enabled.

basenc is a brand-new gnu core utility (added less than 3 years ago!),
which enables some more encodings.

migrate `cat` code to UResult #2464

maint: enable gnu test for arch

Gnu checks before testing a utility whether that utility was actually built.
It thinks `arch` was not built, even though it was.

This patch does not convince gnu that `arch` was built (I wasn't able to figure that out),
but instead comments out the check in `init.cfg` when running `build-gnu.sh`.

base32/64: make the gnu test pass

When decoding an invalid base32/64 string, gnu writes everything it was
able to decode until it hit the decode error, while we don't write
anything if the input is invalid.

Only one operand is allowed, so when there are multiple arguments we
should print the second argument in the error.

tac: support null separator (-s "")

tests: better arg names in link helper functions

id: add support for showing SELinux context (--context/-Z)

* Bump MSRV from 1.43 to 1.47 in order to be able to compile
`selinux` crate 0.1.3

* Bump MSRV for clippy to 1.47

* Add "selinuxlib" to spell-checker

sort: use UResult

Change the argument names in

* `AtPath::hard_link()`,
* `AtPath::symlink_file()`, and
* `AtPath::symlink_dir()`,

from `src` and `dest` to `original` and `link` to match the arguments of
the corresponding functions from the Rust standard library. For example,
see `std::os::unix::fs::symlink()`.

tail: handle `-` as an alias for stdin

sort: handle cases where the output file is also an input file

For multi-threaded programs like sort it is necessary to be able to send
errors between threads.