Support for `localtime_r`

[WhyNotHugo]

Would be great to have support for localtime_r.

[WhyNotHugo]

libc::localtime_r is effectively broken and unusable due to outdated definitions for time_t. See: rust-lang/libc#4307

[sunfishcode]

rustix is scoped to syscall-like functions, and localtime_r is a higher-level function, so this is out of scope for rustix.

The tz-rs crate contains an implementation of localtime_r functionality, which you may find useful.