Add factory functions to distinguish between when the owner of the File
wants to transfer ownership to the new IPC object (adopt) or to send a
copy of the same fd to the IPC peer (clone).
This behavior is more intuitive than the previous behavior. Previously,
an IPC::File would default to a shallow clone of the file descriptor,
only *actually* calling dup(2) for the fd when encoding or it into an
IPC MessageBuffer. Now the dup(2) for the fd is explicit in the clone_fd
factory function.
6d4ba21832