Various fixes for `visudo`

[pvdrz]

Describe the changes done on this pull request This PR does a couple fixes to visudo:

• Lock the sudoers file in nonblocking mode.
• Use /usr/bin/editor as the default editor:
  • Check the SUDO_EDITOR, VISUAL and EDITOR env vars if the default editor is not available.
  • Use vi otherwise (this is temporary as my OS doesn't set /usr/bin/editor and compiles visudo with vi as the default editor).
• Invoke the command passing the temporary file after --.
• Avoid showing the "What now?" message and waiting for input if stdout is not a terminal.

It also fixes a small mistake in the Dockerfile where su was being installed as visudo.

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.
• [x] This pull request will fix parts of #657 where a proper discussion about a solution has taken place.

[github-actions[bot]]

Number of dependencies and binary size impact report

Metric	main	PR #663	Delta
Direct dependencies	3	3	-
Total dependencies	4	4	-
Binary size	979 KiB	983.3 KiB	+0.4%
Text size	571.5 KiB	573.7 KiB	+0.4%

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: 23.63% and project coverage change: -0.21 :warning:

Comparison is base (27b9ade) 59.02% compared to head (b9b454f) 58.81%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #663 +/- ## ========================================== - Coverage 59.02% 58.81% -0.21% ========================================== Files 66 66 Lines 8764 8800 +36 ========================================== + Hits 5173 5176 +3 - Misses 3591 3624 +33 ``` | [Impacted Files](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/663?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety) | Coverage Δ | | |---|---|---| | [src/visudo/mod.rs](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/663?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety#diff-c3JjL3Zpc3Vkby9tb2QucnM=) | `0.00% <0.00%> (ø)` | | | [src/system/timestamp.rs](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/663?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety#diff-c3JjL3N5c3RlbS90aW1lc3RhbXAucnM=) | `80.04% <75.00%> (ø)` | | | [src/system/file.rs](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/663?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety#diff-c3JjL3N5c3RlbS9maWxlLnJz) | `77.04% <76.92%> (ø)` | | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/663/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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