Auto merge of #1387 - tormol:fionclex, r=gnzlbg

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).