run: was getting a little lost

This is a multipoller based on epoll rather than poll.

Note that this implementation is aimed at correctness rather than
performance, although it should immediately have better scalability to
large numbers of FDs, both due to epoll's O(1) sized API and due to not
needing to wake up polling threads to do interest set changes.

One notable difference here is that we directly attach a wakeup fd
rather than using the freelisting kick mechanism that the poll() based
implementations use, because modifying the epoll set to use a different
kick fd each time isn't free.

Also adding windows helpers to convert to and from TCHAR strings.

Caveats:
-) The win32 pollset isn't threadsafe (yet).
-) Only client code is implemented.
-) Only very simple code has been tested with it yet.

- Had to add support for files and environment variables as well.
- I can't compile on windows so I'm sure there will be some issues.
- Tested end-to-end with the simple ssl fullstack test.

Fix the transitive closure of this change, including reintroducing
pollset_kick_posix/windows where the latter is just a stub.

This introduces the wakeup fd interface, corresponding approximately to
the existing Google version, complete with a ported giant detailed usage
comment.

The implementation has two layers, "specialized" and "fallback". The
specialized layer is intended to be a suitable platform specific
implementation like eventfd, whereas "fallback" is probably pipe, with
runtime detection of whether the specialized version works on this
system (currently stubbed out).

This adds support for eventfd based kicking, with the skeleton of
support for runtime selection between eventfds and pipes.