Mask all signals in GLib worker thread

Mask all signals in GLib worker thread

Some code using GLib (gnome-keyring-daemon, for example) assumes that
they can catch signals by masking them out in the main thread and
calling sigwait() from a worker.

The problem is that our new worker thread catches the signals before
sigwait() has a chance and the default action occurs (typically
resulting in program termination).

If we mask all the signals in our worker, then this can't happen.