`mktemp`: inconsistency between Windows and Unix

mktemp uses env::temp_dir to get the default temporary directory. The behaviour of this function is different between platforms, which is confusing for writing cross-platforms scripts (which is supposed to be a strength of uutils!).

Here's what's different:

• On Windows, TMPDIR is ignored.
• On Unix TMP, TEMP and USERPROFILE are ignored.
• On Unix, if TMPDIR is a relative path, a relative path is printed.

So we have a couple of options.

1. Make Windows behave like Unix, which means copying the code from std::env::temp_dir from Unix. This makes it consistent, but a bit weird on Windows, because we ignore standard environment variables.
2. Add a manual check for TMPDIR to the Windows implementation. This helps consistency, but then Unix does not behave like Windows.
3. Add all variables to both Unix and Windows (at least TMPDIR, TMP & TEMP). This is consistent between platforms, but not consistent with GNU.

Any opinions on this?

Start by locating mktemp's use of env::temp_dir and the platform-specific temporary-directory implementations. Compare the Windows and Unix environment-variable behavior described in the issue, then review the discussion to identify an agreed direction. Done requires a decided cross-platform behavior and tests covering the relevant variables and relative paths.