Another use of the deprecated unsafe preexec_fn was to call os.umask in the child prior to exec.

As seen in https://github.com/freeipa/freeipa/pull/3769 (see the code in there).

We should add an explicit feature for this to avoid people's desire for preexec_fn or for the heavyweight workaround of an intermediate shell calling umask before doing another exec.

Any common preexec_fn uses that we can encode into supported parameters will help our ability to remove the ill fated preexec_fn misfeature in the future.