Simplify sudo check on unix

[tesuji]

This PR uses

• cfg(unix) attribute to limit sudo check on Unix platforms only.
• Move unsafe code interact with FFI to separate function: home_dir_from_passwd
  • Use libc::sysconf to get default entry size.

[kinnison]

There's a substantial set of changes in there for "Simplify" but I think I follow them. This kind of rework, particularly when there is unsafe code is much more easily reviewed as a series of commits for future reference.

Could you please detail what testing you've done to ensure this is right, considering that the test code deliberately skips it in almost every case?

Also, how does this interact with work done in #1936 and does it mean that #699 is obsolete?

[tesuji]

Yeah, I'll try to improve that in the future.
About #699, I think e308c56 enabled it again.
This PR doesn't interact with #1936 at all. More info in the rewrote PR description.

[kinnison]

Thank you for splitting things up and clarifying the relationships with those other items.

LGTM.