Signal handling not thread-safe

[sethrj]

Interrupting the Geant4 offload call in a multithreaded Geant4 context causes one thread to register the interrupt, but it needs to be able to pass control to another thread so it can be interrupted too? Or at least it shouldn't clear the bit and should allow all the threads to see the signal. Currently, hitting Ctrl-C twice during transport usually aborts the program.