Prevent exit when a job is suspended

[magicant]

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Many existing shells refuses to exit when they have a suspended job. It would be helpful for yash-rs to support this behavior.

This behavior is extension to POSIX.

Todos:

• [ ] Prevent exit on end of input
• [ ] Prevent exit in the exit built-in
• [ ] Add the -f option to the exit built-in

Questions:

• [ ] Should this happen only if interactive is set?
  • This should never happen in subshells.
• [ ] Should this happen only if monitor is set?