Fix link in README.md

Same as #1417, but actually against the right branch this time.

wasi: add c_schar definition

This fixes an issue when building https://github.com/capstone-rust/capstone-rs/ for WASI.

Otherwise, I get:
~~~
error[E0412]: cannot find type `c_schar` in module `libc`
 --> /home/user/capstone-rs/target/wasm32-wasi/debug/build/capstone-sys-1019f1f8759b0d05/out/capstone.rs:4:27
  |
4 | pub type __int8_t = libc::c_schar;
  |                           ^^^^^^^ help: a type alias with a similar name exists: `c_char`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0412`.
error: Could not compile `capstone-sys`.
~~~

The file with the error is generated with bindgen:

https://github.com/capstone-rust/capstone-rs/blob/a74126cd88d683c0c16029be361ee85c12934878/capstone-sys/pre_generated/capstone.rs#L3-L5

Remove new field from ucontext_t for compatibility with earlier glibc versions

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

Add RTM_* constants to linux/mod.rs for rtnetlink

There is one last set of constants I'm looking to get added to libc for rtnetlink support in my netlink library. I'm going to follow this up with a release PR once this PR is merged.

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