Auto merge of #496 - lucab:to-upstream/fchdir, r=alexcrichton

unix: add fchdir(2)

Add missing [fchdir(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/fchdir.html) function - chdir to an open file descriptor.

Fixes https://github.com/rust-lang/libc/issues/495

src/unix/mod.rs

@@ -366,6 +366,7 @@ extern {
     pub fn access(path: *const c_char, amode: ::c_int) -> ::c_int;
     pub fn alarm(seconds: ::c_uint) -> ::c_uint;
     pub fn chdir(dir: *const c_char) -> ::c_int;
+    pub fn fchdir(dirfd: ::c_int) -> ::c_int;
     pub fn chown(path: *const c_char, uid: uid_t,
                  gid: gid_t) -> ::c_int;
     #[cfg_attr(all(target_os = "macos", target_arch = "x86"),