> Looking at the glibc code, it really and truly opens the directory as a file

Note that opendir() does not portably return a file descriptor. And
AFAIK there's no documented way to get a file handle from the search
handle returned by the WinAPI FindFirstFile() function.

Therefore the SRFI has to treat opendir("/some/dir") and
open("/some/dir") as fundamentally different operations. Hence to avoid
confusion, open-directory and close-directory should be called something
else.

R6RS has the following procedures:

- (get-u8 port)
- (get-bytevector-n port count)
- (get-bytevector-some port)
- (get-bytevector-all port)

We could have:

- (open-directory-list path/port/fd) => handle
- (read-directory handle)
- (read-directory-n handle count)
- (read-directory-some handle count)
- (read-directory-all handle)
- (close-directory-list handle)

It turns out that read-directory-all is basically equivalent to
`directory-files` (if it returns a list instead of a vector - it's up
for debate which type the results should be).

IMHO `read-directory-some` is nice since it's the easiest to optimize
(with regards to buffering and locking) for scenarios where performance
really counts.

As for (open "/some/directory"), Linux and some BSDs have an O_DIRECTORY
flag to ensure that open() only opens a directory, not a regular file.