* chore: Use std::exchange in UnixFd
This was suggested in code review for #376 .
* fix: Protect against UnixFd self-assignment
While self-assignment is rare, it is expected to be safe. Add a check
to prevent putting the object in an invalid state.
* fix: Improve hygiene around dup system call
- Don't try to call dup on a negative value.
- Check dup return code and throw if it fails, rather than returning an
empty UnixFd object.
* chore: Move UnixFd::close to Types.cpp
Minor convenience for applications: unistd.h doesn't have to be included
in the public header.
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Co-authored-by: David Reiss <dreiss@meta.com>