tempfile: add new_with_modes and is_named methods

[champtar]

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On filesystems that do not support O_TMPFILE (e.g. vfat), TempFile::new() automatically falls back to a potentially world readable named temp file.

Add TempFile::new_with_modes() to create temp files with a chosen mode directly. We pass 2 modes, one for unnamed tempfile (happy path, created with O_TMPFILE), and one for named tempfile.

The final mode is still restricted by the process umask.

@cgwalters this will help improve cap-std-ext atomic* permissions handling