Support F_DUPFD on stdin/stdout/stderr
Enable `close`-ing stdin/stdout/stderr
For `dup`, check if FD is `File` first
If not, clone the appropriate standard IO stream
Merge POSIX `close` and `dup` tests into same module
Also, add assertion that `write` on a closed FD returns an error.
Add `dup` as FileDescriptor trait fn
Also:
- Fix `close` so it drops `self` instead of reference to it
- Remove FD clamping in insert_fd_with_min_fd, since FDs 0-2 can be
closed
Fix fs_libc tests
Make error message when closing stdin/out/err more specific
Return io::Result from `FileDescriptor::dup`
Change error message when closing stdin/out/err
Refactor `FileDescriptor::dup` impl for `FileHandle`
Remove empty line
