Fix autoconf test for `diff` flags on macOS

GNU diffutils identifies /dev/zero as identical to /dev/zero by
special-casing, but Apple's diff, for example, does not. Use
/dev/null instead.

Add String.is_empty

* Add String.of_char

* Promote partiality.ml test

* Update changes

Disable the lib-runtime-events/test_create_cursor_failures.ml test when the current user is a superuser

* Enable native backend for DragonFly BSD

Without this, it's not possible to build ocamlopt on DragonFly BSD.

Tested-on: DragonFly 6.5

* Add Changelog entry

* Mention DragonFly has Tier 2 support of native-code compiler

I like mentioning DragonFly in close proximity to the other BSDs,
ideally next to FreeBSD, as it shares most similarities with FreeBSD.
In this case, FreeBSD is in Tier 1, so mention it after OpenBSD. Sorry
for shifting OmniOS to the right.

As of the naming: We call it "DragonFly" and only sometimes "DragonFly
BSD" when talking to non-DragonFly people.

Use more POSIX thread-safe functions in libunix

Fix the testsuite on _all_ CRLF checkouts

Clarify barriers and spin macros with delayed expansion

Cache test results of custom Autoconf tests from `aclocal.m4`

`unixsupport.h`: fix C++ missing `std::` namespace on `atomic_int`

The `std::` namespace was missing from
bdbe619f1655a02c3ce18dcb745a4cf96c672dc2.

- https://pubs.opengroup.org/onlinepubs/9799919799/functions/mktime.html
- https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/mktime-mktime32-mktime64?view=msvc-170
- C11 standard § 7.26.2.3 The mktime function

- https://pubs.opengroup.org/onlinepubs/9799919799/functions/gmtime_r.html
- https://pubs.opengroup.org/onlinepubs/9799919799/functions/localtime_r.html

- https://pubs.opengroup.org/onlinepubs/9799919799/functions/getgrnam_r.html
- https://pubs.opengroup.org/onlinepubs/9799919799/functions/getgrgid_r.html
- https://pubs.opengroup.org/onlinepubs/9799919799/functions/getpwnam_r.html
- https://pubs.opengroup.org/onlinepubs/9799919799/functions/getpwuid_r.html

Configure fixes for Windows

Open the file in binary mode on all platforms and simply ignore \r
characters when counting the total number of characters.

mnemonics.mll opens the channel in text mode, which on Windows
automatically transforms the line-endings. Tweak the test so that the
behaviour is uniform on both Unix and Windows.