Auto merge of #406 - philippkeller:shadowpw, r=alexcrichton

Add shadow password bindings

Add shadow password bindings: setspent, endspent, getspent, getspnam plus spwd, the corresponding struct.

[Example code](https://github.com/philippkeller/apue-rust/blob/master/src/bin/e06-01-shadow-pw.rs) (tested on Linux 4.4.0-21-generic)