Signal handling in libafl_bolts is very unsafe

[addisoncrump]

There's a lot of potential for double-mutability here with &mut self in handle. We should either take the reference when a signal is being handled and replace it when finished or use &self.

[domenukk]

Signals don't really "finish", all signal handling we do (not in bolts, but in their respective users) will quit execution after the signal handler, so at least they never borrow mutably twice. The mutable reference moves into the signal handler in this case.