linux: TIOCGPTPEER support

At the moment the `NewPty` API is based around the `ptsname`-style API where you get a pathname in `/dev/pts/` that you can open. However, [many years ago I added support for getting the peer end of a pty with the `TIOCGPTPEER` ioctl](https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=54ebbfb1603415d9953c150535850d30609ef077) (allowing you to get the peer end without worrying about the filesystem having a fake `/dev/pts`).

In principle, you can do this manually today, but it would be nice if this was supported (on Linux at least). #86 adds `GetPtyFromFile`, maybe `GetPtyFiles` would be better? (On other operating systems you could just return the non-race-free