Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Use the GMainLoop specific function to wake up the main loop.  This is
simpler and comes with very little overhead.

Don't bother checking for the parameter mask.  This doesn't matter.
The only thing that matters is if a wake-up is already pending.

Less overhead than fetch_and() for what we do.

Pipes aren't really pollable on Windows.
GLib emulates polling on pipes using separate reader/writer threads.
This adds significant overhead and keeps a strong dependency on GLib.

socketpair() function is absent on Windows as well.
We implement it here in a loose way.

Use a bit field instead of a mutex-protected bool array.