Warn the end user when they enter potentially dangerous option values

[nemec]

There may be some scenarios where you want to warn the end user if they enter a dangerous value that they may not have indended, the canonical example being rm -rf / bin/someprog and putting an extra space in there.

Today the rm program uses an extra parameter (--no-preserve-root) to allow that value, but it also terminates the program immediately with an error. This feature will offer devs the option to choose "warn", "error", or custom behavior. To get the behavior used by rm today, you would have to implement your own post-parse validation.