Auto merge of #307 - darnuria:relative-directory-posix2008-api, r=alexcrichton

Add Directory file descriptor relative syscall.

Add Directory file descriptor relative syscall.

I only added "*at" syscall relative to Posix2008 specification.

At the moment OSX and Android are partially supporting this API.

This include:
* `openat`
* `faccessat`
* `fchmodat`
* `fchownat`
* `fstatat`
* `linkat`
* `mkdirat`
* `mknodat` [0]
* `readlinkat`
* `renameat`
* `symlinkat`
* `unlinkat`
* `mkfifoat` [0][1]

[1] Not available on Android at the moment:
* `mkfifo`

[1] Not available on OSX at the moment:
* `mkfifoat`
* `mknodat`

And exclude operating system specific API:
* `renameat2`: Linux specific
* `execveat`: Linux specific since 3.19
* `futimesat`: GNU extension
* `name_to_handle_at`: GNU extension
* `scandirat`: GNU extension
* `fanotify_mark`: belong to fanotify API.