TLS handling

[tromey]

There doesn't seem to be a good way to deal with TLS. It doesn't matter hugely since Emacs is single-threaded, but at least errno relies on TLS and we need a way to expose that.

[tromey]

Emacs isn't single-threaded any more.

Errno could be handled specially by the library. That wouldn't be totally unreasonable.

If other libraries export TLS symbols then we'd need to think of something tricky.