Add _SC_NPROCESSORS_ONLN for CloudABI.

This constant is not used by the C library, which is why I didn't add it
initially. It is, however, used by libtest to determine the parallelism
for the execution of tests.

This constant is not used by the C library, which is why I didn't add it
initially. It is, however, used by libtest to determine the parallelism
for the execution of tests.

Add support for CloudABI.

CloudABI is a sandboxed UNIX-like runtime environment, based on the
principle of capability-based security. As CloudABI is intended to be
cross-platform, the system call layer is specified here:

    https://github.com/NuxiNL/cloudabi/blob/master/cloudabi.txt

From these definitions, we automatically generate C and Rust bindings.
The latter is published on crates.io:

    https://crates.io/crates/cloudabi

My goal is to implement libstd for CloudABI in such a way that it uses
the C library as little as possible; only in places where it would ease
interfacing with C code (e.g., thread creation). In places where
constants in the C library are directly based on the CloudABI
specification (e.g., errnos), use the constants provided by the cloudabi
crate.

These are now used by libstd's termination.rs.

Add some constant and struct on macOS

file: `net/bpf.h`

CloudABI is a sandboxed UNIX-like runtime environment, based on the
principle of capability-based security. As CloudABI is intended to be
cross-platform, the system call layer is specified here:

    https://github.com/NuxiNL/cloudabi/blob/master/cloudabi.txt

From these definitions, we automatically generate C and Rust bindings.
The latter is published on crates.io:

    https://crates.io/crates/cloudabi

My goal is to implement libstd for CloudABI in such a way that it uses
the C library as little as possible; only in places where it would ease
interfacing with C code (e.g., thread creation). In places where
constants in the C library are directly based on the CloudABI
specification (e.g., errnos), use the constants provided by the cloudabi
crate.

Add some constants for linux and macos

macos:
```
netinet/if_ether.h
net/route.h
```

linux:
```
linux/if_ether.h
```

Add SECCOMP_MODE_* flags on Linux

Change deprecated constants into hidden constants

rustc, an important libc consumer, has a policy that they can't use any
crates with deprecated symbols.  Replace libc's two deprecated symbols
with hidden symbols instead.

rustc, an important libc consumer, has a policy that they can't use any
crates with deprecated symbols.  Replace libc's two deprecated symbols
with hidden symbols instead.

Add additional aarch64 linux syscalls

I've noticed some syscalls are missing for aarch64 linux, I've added those I needed. The numbers have been taken from a raspberry pi 3.

Adds:
- `SYS_fcntl`
- `SYS_lseek`
- `SYS_newfstatat`
- `SYS_fstat`
- `SYS_mmap`
- `SYS_open`
- `SYS_stat`
- `SYS_lstat`
- `SYS_pipe`
- `SYS_getdents`
- `SYS_poll`

POSIX mqueue bindings for the BSDs

Note that OpenBSD and OSX do not support POSIX message queues.

Note that OpenBSD and OSX do not support POSIX message queues.

Add `net/if_utun.h` and `linux/if_tun.h` constants for macos and linux platform

Add `net/if_utun.h` constants for macos platform.

Add `linux/if_tun.h` constants for linux platform.

Openbsd aarch64

it adds aarch64-unknown-openbsd definition (and push `c_char` deeper, so the amount of changes).

I also updated errno definitions for openbsd

move c_char definition more deeper to make it 'u8' on
aarch64-unknown-openbsd.

Add struct `sockaddr_dl` for bsdlike platforms

Freebsd: https://github.com/freebsd/freebsd/blob/master/sys/net/if_dl.h#L59
Macos: https://github.com/apple/darwin-xnu/blob/master/bsd/net/if_dl.h#L95

Add missing EAI_* flags for multiple platforms

Add missing EAI_* flags for multiple platforms.

Ucreds

I'm not positive on the `SOCKCREDSIZE()` implementation, but we'll see what CI says I guess.