Calling `unwrap` on the result of `CString::new` may cause the current
thread to panic, which is a bit surprising undocumented behavior. It
would be more reasonable to treat the erroneous name as a non-existing
user or group.

When reading a value into an enum from getsockopt, we must validate it.
Failing to do so can lead to UB for example with SOCK_PACKET on Linux.

Perform the validation in GetSockOpt::get.  Currently SockType is the
only type that requires validation.

Fixes nix-rust#1819

This is a backport of 006fc6f. The
original commit message follows:

In a future release of the `libc` crate, `libc::timespec` will contain
private padding fields on `*-linux-musl` targets and so the struct will
no longer be able to be created using the literal initialization syntax.

Update places where `libc::timespec` is created to first zero initialize
the value and then update the `tv_sec` and `tv_nsec` fields manually.
Many of these places are in `const fn`s so a helper function
`zero_init_timespec()` is introduced to help with this as
`std::mem::MaybeUninit::zeroed()` is not a `const` function.

Some matches on `libc::timespec` are also updated to include a trailing
`..` pattern which works when `libc::timespec` has additional, private
fields as well as when it does not (like for
`x86_64-unknown-linux-gnu`).

It just so happens that Redox, OpenBSD, Dragonfly, and uclibc don't use
some of the rules for two internal macros.

Clippy is now smarter about detecting unnecessary casts and
useless conversions, which means we need to be more explicit
about when the conversions are needed for a subset of platforms.

Required changes found by repeatedly running the following command
against a list of the supported platforms.

`xargs -t -I {} sh -c "cargo clippy -Zbuild-std --target {} --all-targets -- -D warnings || exit 255"`

I removed the casts it complained about, and then restored them
with an `#[allow]` if a later target needed the cast.

.

Signed-off-by: Alex Saveau <saveau.alexandre@gmail.com>

Due to rust-lang/rust#97030, cargo test will fail to doctest macros
unless they are exported, breaking the examples for libc_bitflags! and
libc_enum!.

Adds `ignore` to the examples for these macros to stop tests from
failing.

The latest redox-syscall crate requires at least Rust 1.59.0, but they
don't define an MSRV policy.  And the version given in the
rust-toolchain file in the Redox repository doesn't work.  So until they
clarify their MSRV, use nightly.

https://gitlab.redox-os.org/redox-os/syscall/-/commit/30f29c32952343412bb6c36c9fda136d26e9431f