Export gdb.block_signals and create gdb.Thread

While working on an experiment, I realized that I needed the DAP
block_signals function.  I figured other developers may need it as
well, so this patch moves it from DAP to the gdb module and exports
it.

I also added a new subclass of threading.Thread that ensures that
signals are blocked in the new thread.

Finally, this patch slightly rearranges the documentation so that
gdb-side threading issues and functions are all discussed in a single
node.