Auto merge of #489 - zackw:add-waitid, r=alexcrichton

Add waitid and related constants and types.

[`waitid`](http://pubs.opengroup.org/onlinepubs/9699919799/functions/waitid.html) is a variation on `waitpid` with a marginally more convenient way of reporting the status, and a couple of handy
additional features, such as the ability to peek at an exit status without consuming it.  It's in POSIX.1-2008 and should be available on all supported Unixes.

Along with it come the type `idtype_t` and the constants `WEXITED`, `WSTOPPED`, `WCONTINUED`, and `WNOWAIT`.  The constants were already defined for unix/notbsd platforms.

The patch is currently incomplete: I'm pushing it to get CI to test platforms I don't have.  Todo list is

* [x] Add a definition of `siginfo_t` to all platforms that don't have it.
* [x] Verify that the new constants are consistent for all \*BSD platforms.
* [x] Verify that `idtype_t` is consistent across the board.
* [x] Add `link_name` annotations for `waitid` if/as necessary.