IMHO the names `port->fd` and `port-internal-fd` are slightly
misleading, though the functionality is good.

The name `port->fd` doesn't obviously say that it creates a copy of the
fd. RnRS has procedures named like `list-copy` and `string-copy` which
create copies of things. Common Lisp has `copy-list`, `copy-seq`, etc.
Calling it `file-descriptor-copy` or `copy-file-descriptor` would match
these known names.

`port-internal-fd` is basically right, but the word `internal` may be
superfluous. `port-fd` or `port-file-descriptor` would say as much. Unix
users will understand that most ports have an underlying fd. If the
intent of `internal` is to say "danger, think twice before using", using
a longer name would probably serve the same purpose?