Add illumos 3socket flags

This adds missing flags for SOCKET(3SOCKET).

Add two missing pthread calls to the OpenBSD support.

These are shamelessly copied from the NetBSD support, whose prototypes in this regard are identical to OpenBSD's.

Add a few functions & POSIX error codes to Windows API

This change adds a few simple additional functions to the windows apis, as well as some additional error codes.

Tests and style pass, as per contributing guidelines.

ci: switch mirrors to use our CDN

We recently added a CDN in front of our CI mirrors as it's faster and
cheaper for us. This switches libc's CI to use it instead of accessing
the underlying bucket directly.

r? @gnzlbg

Add struct ip_mreqn: ip multicast req with intf id

The ip_mreqn struct has an additional interface id parameter. This
allows programming IPv4 multicast groups using the interface id
alongside the src IPv4 address.
Linux has supported the ip_mreqn struct since Linux 2.2
ref: man ip.7

Add extended IFLA_ consts

Only Netlink interface link attributes up `IFLA_STATS` are currently available. This commit adds `IFLA_COST` to `IFLA_MAX_MTU`.

See https://github.com/torvalds/linux/blob/54ecb8f7/include/uapi/linux/if_link.h#L106

Thanks!

Add support for making functions `const`

PR https://github.com/rust-lang/rust/pull/64906 adds the ability to write `const extern fn` and `const unsafe extern fn`, which will allow manys functions in `libc` to become `const`.

This is particuarly useful for functions which correspond to C macros (e.g. `CMSG_SPACE`). In C, these macros are constant expressions, allowing them to be used when declaring arrays. However, since the corresponding `libc` functions are not `const`, writing equivalent Rust code is impossible. Users must either perform an unecessary heap allocation, or pull in `bindgen` to evaluate the macro for specific values (e.g. `CMSG_SPACE(1)`).

However, the syntax `const extern fn` is not currently parsed by rust. To allow libc to use this without breaking backwards compatibility (i.e. bumping the minimum Rust version), I've taken the following approach:

1. A new off-by-default feature `extern-const-fn` is added to `libc`.
2. The internal `f!` macro has two versions, selected at compile-time by a `cfg_if`. When `extern-const-fn` is enabled, the declared `f!` macro passes through the `const` keyword from the macro user to the final definition (`pub const unsafe extern fn foo`. When  `extern-const-fn` is disabled, the `const` keyword passed by the macro user is discarded, resulting in a plain `pub extern const fn` being declared.

Unfortunately, I couldn't manage to get `macro_rules` to accept a normal `const` token in the proper place (after `pub`). I had to resort to placing it in curly brackets:

```rust
pub {const} fn foo(val: u8) -> i8 {
}
```

The `f!` macro then translates this to a function definition with `const` in the proper position.

I'd appreciate it if someone who's more familiar with `macro_rules!` could see if I missed a way to get the desired syntax.

add shm support for NetBSD and OpenBSD

initial work from @landryb for OpenBSD, various fixes and NetBSD support from me.

Fixes #1585

shm support is need for firefox building (for slice-deque)

cc @jakllsch @ryoon for NetBSD

Minor Readme update

Deprecate vfork

The compiler may generate incorrect code for `vfork` and `setjmp` because they are missing the `#[returns_twice]` attribute which is currently unstable ([tracking issue](https://github.com/rust-lang/rust/issues/58314)). Since `vfork` is impossible to use safely, I propose deprecating it until `#[returns_twice]` is stable.

Disable broken targets

Only Netlink interface link attributes up IFLA_STATS are currently
available. This commit adds IFLA_COST to IFLA_PROTO_DOWN.

See https://github.com/torvalds/linux/blob/54ecb8f7/include/uapi/linux/if_link.h#L106

initial work from @landryb for OpenBSD, various fixes and NetBSD support from me.

Fixes #1585

Co-Authored-By: gnzlbg <gnzlbg@users.noreply.github.com>

Add a new feature to enable this, since `const extern fn`
support is unstable

We recently added a CDN in front of our CI mirrors as it's faster and
cheaper for us. This switches libc's CI to use it instead of accessing
the underlying bucket directly.

Remove duplicate target

The ip_mreqn struct has an additional interface id parameter. This
allows programming IPv4 multicast groups using the interface id
alongside the src IPv4 address.
Linux has supported the ip_mreqn struct since Linux 2.2
ref: man ip.7

[Adding to linux x86_64 only as adding to linux_like/mod.rs caused
failures in the sparc64 and mips64 C regression tests]

Add settimeofday for solaris/illumos

Closes #1547

Sort targets in documentation. Fixes #1524.

Should be a simple but effective fix for #1524. Assume `sort` command is available.

Should be a simple but effective fix for #1524. Assume `sort` command is available.

Refine definition of mcontext_t on x86_64-unknown-linux-musl

Fixes #1553

I've inlined the links I used for defining constants and fields, although they pretty much map to the changes introduced in ac7f0fac and ae06c2b2.