[PATCH] Add "scandir" procedure

[nalaginrut]

Hi guys!
I found there isn't "scandir" in current Guile. And we may use "ftw" to
instead. I guess "ftw" traverse all sub-directoies. Yes, we may use nftw
to filter the level we don't need, but "ftw" seems always traverse all
sub-directories. If my guess is correct, I believe it's too slow for
someone, for a instance, me. :-)
But the efficiency is not the case.
The reason is that when I was trying to write a web server ,I need to
print directory content in HTML back to the client. And I found "ftw"
won't return "." and "..", moreover ,the result isn't sorted.
And I think "scandir" which provided by POSIX is a better solution. Many
dynamic language like PHP own a "scandir" implementation. I believe
Guile should have one. So I wrote an implementation into filesys.c.

There's a little difference of usage between C and Guile version. But
it's trivial if you're familiar with the C version.

The usage is like this:
-------------------------------
scandir dir [filter [sort]]
-------------------------------

"dir" must be a string which owned by a directory. You may run with one
arg directly:
------------------
(scandir "mmr")
==>("." ".." "a" "b" "c" "dd" "dir1" "dir2")
------------------

As you see, it returned a list contained all contents of directory
"mmr". So, you can do whatever you like with a list.

And you may use filter:
------------------------
(scandir "mmr"
   (lambda (fname)
       (if (string-contains fname "dir")
             #f
             #t)))
==> ("." ".." "a" "b" "c" "dd")
------------------------
It's easy to filter the file you want. Return #f will skip current file,
#t will add it to the result list.

And the third arg "sort" has only two possibility: 
'asort stands for alphasort
or
'vsort stands for versionsort
Call "scandir" with "sort" unspecified you'll get 'asort in default. If
you can't understand that, you may checkout the C version manual: 
----------------
man 3 scandir
----------------

Besides, there's a clumsy documentation in the REPL:
-------------
,d scandir
-------------

PS: There must be some bugs in this implementation, like portable
problem. I believe I didn't add some critical macro in it. But I think
there're lots of people know more than me in the mail-list. So I submit
it, and waiting for any advices or patches of patch. :-)
Anyway, I've tested it. It's function complete. Use it as you like!

My patch will be submitted in the next mail, I found some guy(like me)
doesn't like to open an attachment of obscure origin. ;-)


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