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.

Since it has variable length, the user of a sockaddr_un must keep track
of its true length.  On the BSDs, this is handled by the builtin sun_len
field.  But on Linux-like operating systems it isn't.  Fix this bug by
explicitly tracking it for SockaddrStorage just like we already do for
UnixAddr.

Fixes nix-rust#1866

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`).

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>