Raise MSRV to 1.69.0

[asomers]

This allows us to use the very useful CStr::from_bytes_until_nul

[SteveLauC]

The [..] in &buf[..] seems to be unnecessary here

[SteveLauC]

The MSRV in README.md needs to be updated as well:

nix is supported on Rust 1.65 and higher. Its MSRV will not be changed in the future without bumping the major or minor version.

BTW, it seems that the version of the next release would be 0.28.0 since we bumped our MSRV in it

[asomers]

BTW, it seems that the version of the next release would be 0.28.0 since we bumped our MSRV in it

We would have to do that anyway, since we've changed the signatures of several functions to add I/O safety.

[SteveLauC]

The CI failure of Fuchsia x86_64:

rustup target add $TARGET
error: toolchain '1.69.0-x86_64-unknown-linux-gnu' does not contain component 'rust-std' for target 'x86_64-fuchsia'
note: not all platforms have the standard library pre-compiled: https://doc.rust-lang.org/nightly/rustc/platform-support.html

seems to be because that the alias x86_64-fuchsia for x86_64-unknown-fuchsia has been removed?

Platform support: tier2

$ rustup target list | grep -i fuchsia
aarch64-unknown-fuchsia
x86_64-unknown-fuchsia (installed)

$ rustup target add x86_64-fuchsia
error: toolchain '1.69.0-x86_64-unknown-linux-gnu' does not contain component 'rust-std' for target 'x86_64-fuchsia'
note: not all platforms have the standard library pre-compiled: https://doc.rust-lang.org/nightly/rustc/platform-support.html

$ rustup target add x86_64-unknown-fuchsia
info: component 'rust-std' for target 'x86_64-unknown-fuchsia' is up to date

[SteveLauC]

To fix the CI failure in Redox x86_64, importing std::ffi::OsStr for all the platforms:

    Checking nix v0.27.1 (/tmp/cirrus-ci-build)
error[E0433]: failed to resolve: use of undeclared type `OsStr`
    --> src/unistd.rs:3932:18
     |
3932 |         .map(|s| OsStr::from_bytes(s.to_bytes()).into())
     |                  ^^^^^ use of undeclared type `OsStr`
     |
help: consider importing one of these items
     |
3    + use crate::OsStr;
     |
3    + use std::ffi::OsStr;
     |

// old one
use std::ffi::{CStr, OsString};
#[cfg(not(target_os = "redox"))]
use std::ffi::{CString, OsStr};

// new one, the CI failure should be resolved by this
use std::ffi::{CStr, OsStr, OsString};
#[cfg(not(target_os = "redox"))]
use std::ffi::CString;