fix: deadlock in supervisor signal handler

Only a few functions are allowed in signal handlers (see `man 7
signal-safety`). Both `println!()` and `std::process::exit()` are not
acceptable. In our case `std::process::exit()` resulted in a dead lock
when called concurrently.

This patch fixes it, and also enhances some other aspects of the supervisor.

See also: https://stackoverflow.com/questions/44865737/println-panics-when-called-concurrently