Convert daemon.Mainloop to @SignalHandled

This makes the function a lot simpler, since it used to install two
different signal handlers, which are now transparently handled by the
decorator. The code is unindented, but remains unchanged apart from the
part that checks the signal handlers (which is now collapsed toghether).

The option of not installing signal handlers, which was unused, is now
removed.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>