Per discussion in #1410, this is necessary to avoid struct size mismatches between Rust and C on
systems with glibc < 2.28.

Support calls to the SunOS Doors API

Doors are a lightweight IPC mechanism available in libc on Solaris & illumos. They are like unix domain sockets, but faster and more pleasant to work with.

* Brief introduction: ["Doors" in SolarisTM: Lightweight RPC using File Descriptors](http://www.kohala.com/start/papers.others/doors.html)
* Relevant manual pages: [DOOR_CALL(3C)](https://illumos.org/man/3C/door_call), [DOOR_CREATE(3C)](https://illumos.org/man/3C/door_create)
* Tutorial I wrote: ["Revolving Doors": A tutorial on the Illumos Doors API](https://github.com/robertdfrench/revolving-door)

Marking this as a draft until I have included the full api.

Add ttyname_r

I hope this is correct. Parameter types taken from `man ttyname`.

```c
int ttyname_r(int fd, char *buf, size_t buflen);
```

Add cfg_attr

haiku: add missing ioctl identifiers to control the TTY

Ref. [termios.h](https://github.com/haiku/haiku/blob/37761761a0c93a96c47d7cfb72f80ab56a8c1722/headers/posix/termios.h#L165-L189)

Add getrandom() bindings on Linux

Closes #659

Remove Linux constants from Fushia

PR #849 just moved all of the Linux structures into `src/fuchsia`.
While this is good for the most part, we really don't want the `SYS_*`
constants exported on Fushia, as these are not Fuchsia's syscalls.

This also removes Linux-specific `GRND_*` constants.

Closes #659

PR #849 just moved all of the Linux structures into `src/fuchsia`.
While this is good for the most part, we really don't want the `SYS_*`
constants exported on Fushia, as these are not Fuchsia's syscalls.

This also removes Linux-specific `GRND_*` constants.

Add FIONCLEX and other FIO* constants for Linux, DragonFly and OpenBSD

[OpenBSD header](https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/sys/sys/filio.h?annotate=1.5) | [DragonFly header](http://gitweb.dragonflybsd.org/dragonfly.git/blame/refs/heads/master:/sys/sys/filio.h) | [Linux search](https://github.com/torvalds/linux/search?q=FIONCLEX+FIOQSIZE&unscoped_q=FIONCLEX+FIOQSIZE&type=Code)
For Linux I've used [godbolt compiler explorer](https://godbolt.org/z/h-C7qM) to get final values where possible.

Wile this crate has FIOCLEX for Fuchsia, Emscripten and Redox, I've not added FIONCLEX for them because those targets appear to support neither.
I started adding FIOASYNC, FIOSETOWN and FIOGETOWN for all the Linux architectures, but gave up when I realized FIOASYNC is kind of deprecated and the other two useless without more symbols. If anybody is interested I have a [branch with how far I got](https://github.com/tormol/rust-libc/tree/sigio).

Switch to manual trait impls for sigevent

`sigevent` on most platforms have padding or unused fields. Rather
than display those in the `Debug` impl by deriving it, manually implement
all `extra_traits` instead ignoring those fields.

I do worry that my `PartialEq` implementations for this for some platforms (like Linux) is not correct due to ignoring bytes that shouldn't be ignored because these structs don't have a proper union set up.

cc @asomers
Part of nix-rust/nix#1035

sigevent structs on most platforms have padding or unused fields. Rather
than display those in the Debug impl by deriving it, manually implement
all extra_traits instead ignoring those fields.

Fix libstd compilation for arm-uclibc target

linux: add VSOCK struct and constants

Add struct sockaddr_vm and VMADDR_ constants to use VSOCK on Linux.
VSOCK is present since Linux 3.9

* Change the type of FIONCLEX on apple platforms from c_uint to c_ulong
* Add FIONCLEX, FIONREAD, FIOASYNC, FIOSETOWN and FIOGETOWN
  for DragonFly and OpenBSD

Re-add PTRACE_DETACH for mips64 GNU targets

During the refactor in 4bd419eb, `PTRACE_DETACH` was accidentally removed on
these targets. This re-adds it.

Fixes brokenness encountered during nix-rust/nix#1035

CC @asomers

During the refactor in 4bd419eb, PTRACE_DETACH was accidentally removed on
these targets. This re-adds it.

Add more posix message queue functions for Linux and all for solarish

I'm not able to run the tests locally (likely due to too old headers on Ubuntu 18.04), but I've tested my [posixmq crate](https://github.com/tormol/posixmq/tree/0decb7c82dd472024c7e90fd8121671881a81ae2) with these functions on Linux and OmniOS.

Refactor the GNU module