Provide a method call to re-initialize rbtrace

[eric]

It would be helpful if there were a simple way to reinitialize signal handlers (and anything else that is needed after forking) to work easier with things like passenger.

Passenger provides a callback that can be used after the signal handlers have been reset.

[SamSaffron]

is this even still an issue? we use rbtrace with unicorn and I am able to attach to the various forked processes just fine