Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>

Note that termios2 doesn't exist on powerpc(64), termios
is used instead.

Rebase of #610 and also move emscripten up much higher in the hierarchy to
ensure that it doesn't have too much of a ripple effect on other platforms.

This involved moving down a good number of definitions, but hopefully was done
with care to not break anything!

It needs to be skipped during testing because there's no way to include
a combination of headers that will expose it without giving a "type has
already been defined" error.