Add support for dup/dup2

[lorentey]

This adds support for duplicating file descriptors via a FileDescriptor.duplicate method:

extension FileDescriptor {
  public func duplicate(
      as target: FileDescriptor? = nil,
      retryOnInterrupt: Bool = true
    ) throws -> FileDescriptor
}

The method dispatches to either dup or dup2 based on whether or not it has an explicit target.

Usually, a non-nil target will be one of the file descriptors conventionally reserved for the stdin/stdout/stderr role; these were previously assigned symbolic names in #21. To target other file descriptors, users will need to use the .init(rawValue:) initializer. (This will rarely be needed, but some like to pass additional communication channels down to child processes using some custom numbering convention beyond the universal 0/1/2.)

let controlFd = try fd.duplicate(as: FileDescriptor(rawValue: 3))

[milseman]

LGTM!