Auto merge of #1350 - dbrgn:ttyname_r, r=gnzlbg

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);
```

Author: bors

src/unix/mod.rs

@@ -754,6 +754,10 @@ extern {
     pub fn tcgetpgrp(fd: ::c_int) -> pid_t;
     pub fn tcsetpgrp(fd: ::c_int, pgrp: ::pid_t) -> ::c_int;
     pub fn ttyname(fd: ::c_int) -> *mut c_char;
+    #[cfg_attr(all(target_os = "macos", target_arch = "x86"),
+               link_name = "ttyname_r$UNIX2003")]
+    pub fn ttyname_r(fd: ::c_int,
+                     buf: *mut c_char, buflen: ::size_t) -> ::c_int;
     pub fn unlink(c: *const c_char) -> ::c_int;
     #[cfg_attr(all(target_os = "macos", target_arch = "x86"),
                link_name = "wait$UNIX2003")]