Fix #1631

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

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

Solarish: missing declaration of port_send and port_sendn

Add `CLOCK_TAI` to linux_like/mod.rs

Add MINSIGSTKSZ and SIGSTKSZ definitions for MacOS