Delay exit as much as possible

[pvdrz]

Describe the changes done on this pull request This PR replaces all the individual calls to exit for child processes for a single call at the top of the exec module. This will guarantee that any internal value created by the exec module will be properly dropped.

It would be possible to move this call to exit upper so other parts of sudo can properly drop any values that should be dropped according to their internal logic.

It also replaces the calls to exit for _exit for three reasons:

• That's what ogsudo does.
• The child processes shouldn't try to flush the parent's IO streams.
• It seems to get rid of the spurious coverage errors at the cost of having 0% coverage in any forked process given that any at_exit callback is not run.

cc @japaric

Pull Request Checklist

• [x] I have read and accepted the code of conduct for this project.
• [x] I have tested, formatted and ran clippy over my changes.
• [x] I have commented and documented my changes.
• [ ] This pull request will fix issue https://github.com/memorysafety/sudo-rs/issues/<#issue> where a proper discussion about a solution has taken place.

[github-actions[bot]]

Number of dependencies and binary size impact report

Metric	main	PR #643	Delta
Direct dependencies	3	3	-
Total dependencies	4	4	-
Binary size	978.5 KiB	978.6 KiB	-
Text size	570.7 KiB	571.1 KiB	-

Dependencies diff

```diff └─ sudo-rs [v0.2.0-dev.20230703] ├─ glob [v0.3.1] ├─ libc [v0.2.147] └─ log [v0.4.19] ```

[codecov[bot]]

Codecov Report

Patch coverage: 51.16% and project coverage change: -10.77 :warning:

Comparison is base (729d1b0) 86.32% compared to head (31b90d5) 75.56%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #643 +/- ## =========================================== - Coverage 86.32% 75.56% -10.77% =========================================== Files 64 58 -6 Lines 8680 5975 -2705 =========================================== - Hits 7493 4515 -2978 - Misses 1187 1460 +273 ``` | [Impacted Files](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/643?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety) | Coverage Δ | | |---|---|---| | [src/exec/use\_pty/monitor.rs](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/643?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety#diff-c3JjL2V4ZWMvdXNlX3B0eS9tb25pdG9yLnJz) | `0.00% <0.00%> (-52.37%)` | :arrow_down: | | [src/system/mod.rs](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/643?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety#diff-c3JjL3N5c3RlbS9tb2QucnM=) | `75.06% <0.00%> (-14.50%)` | :arrow_down: | | [src/exec/use\_pty/parent.rs](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/643?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety#diff-c3JjL2V4ZWMvdXNlX3B0eS9wYXJlbnQucnM=) | `69.49% <23.52%> (-3.89%)` | :arrow_down: | | [src/exec/mod.rs](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/643?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety#diff-c3JjL2V4ZWMvbW9kLnJz) | `77.27% <83.33%> (-9.77%)` | :arrow_down: | | [src/exec/no\_pty.rs](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/643?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety#diff-c3JjL2V4ZWMvbm9fcHR5LnJz) | `64.60% <88.88%> (-5.62%)` | :arrow_down: | | [src/sudo/pipeline.rs](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/643?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety#diff-c3JjL3N1ZG8vcGlwZWxpbmUucnM=) | `84.86% <100.00%> (+0.20%)` | :arrow_up: | ... and [40 files with indirect coverage changes](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/643/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety)

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