unistd: adding linux(glibc)/freebsd close_range.

[devnexen]

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Allows to close a range of file descriptors, can set close-on-exec on these and/or unsharing (as having its own file descriptors table after fork for the calling process).

What does this PR do

Checklist:

• [x] I have read CONTRIBUTING.md
• [x] I have written necessary tests and rustdoc comments
• [x] A change log has been added if this PR modifies nix's API