This enables assignment through triagebot on this repository, in preparation for
the migration from highfive to triagebot for PR assignment.

Add Lockf POSIX to UNIX targets

Add accept4() for NetBSD, Illumos and Solaris

Add copy_file_range to linux_like

Fix #1631

This is a Linux-only feature that was already present as a
syscall. Add it just to linux/gnu for the moment, as the musl version
bundled in the Rust's x86_64-unknown-linux-musl toolchain doesn't
include it yet.

Signed-off-by: Sergio Lopez <slp@redhat.com>

ci: allow overriding run-docker.sh CARGO_HOME.

The `ci/run-docker.sh` utility script adds a `--volume` argument to the
`docker` command to mount the Cargo home directory of the host machine
into the container at `/cargo`.

Prior to this patch the host's Cargo home directory is assumed to be the
`dirname` of the `dirname` of the `cargo` command's path. That works in
most cases where the host machine installed rust with vanilla
`rustup`. It may fail if the host machine used a different method.

For example if the host machine used the Archlinux rustup package[0]
then `cargo` is installed to `/usr/bin/cargo` and the `run-docker.sh`
script incorrectly mounts `/usr/` to the `/cargo` directory of the test
container.

This patch allows specifying an explicit `CARGO_HOME` to the
`ci/run-docker.sh` script so that users with a non-standard cargo dir
can use the utility without modification. By default if no `CARGO_HOME`
is set then the legacy behaviour is used and `CARGO_HOME` defaults to
the `dirname` of the `dirname` of the `cargo` command is used.

[0]: https://wiki.archlinux.org/index.php/rust#Arch_Linux_package

References:
* NetBSD (became available with 8.0):
  http://cvsweb.netbsd.org/bsdweb.cgi/src/sys/sys/socket.h?annotate=1.129&only_with_tag=MAIN
  http://cvsweb.netbsd.org/bsdweb.cgi/src/lib/libc/sys/accept4.c?annotate=1.2&only_with_tag=MAIN
* Illumos:
  https://illumos.org/man/3socket/accept
  https://github.com/illumos/illumos-gate/blob/master/usr/src/uts/common/sys/socket.h
  https://github.com/illumos/illumos-gate/blob/master/usr/src/lib/libsocket/socket/weaks.c
* Solaris:
  https://docs.oracle.com/cd/E88353_01/html/E37843/accept-3c.html

Add definition of ucontext_t for macOS

Remove MSVC's wmemchr declaration

Bump version to 0.2.68

Add constants from `linux/in.h`

Add reenterant shadow.h functions

Enable `s390x-unknown-linux-gnu` on CI

Tweak `MAP_SYNC`

Add missing AT_ constants

Add AT_EACCESS to Linux, Solaris, and Fuchsia.

Add AT_SYMLINK_FOLLOW, AT_REMOVEDIR, and  _AT_TRIGGER to Solaris.

Unify mmap flags

add macOS mount flags

https://github.com/apple/darwin-xnu/blob/master/bsd/sys/mount.h#L288

Reduce checks for demoted apple 32-bit targets

Add F_OFD_* constants

Update badges metadata

Add constants from linux/keyctl.h

Move linux/reboot.h constants into linux module