Too many open files

I get Too many open files running `sig make-all -r -i /var/www/gallery/albums/` on my gallery.
My ulimits is set to 1024 files, which I think should be enough for most tasks.

Perhaps sig forgets to close files after use?

```
Creating gallery from images in /var/www/gallery/albums/
.............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................Backtrace:
In ice-9/boot-9.scm:
222:29 19 (map1 _)
222:29 18 (map1 _)
222:29 17 (map1 _)
222:29 16 (map1 _)
222:29 15 (map1 _)
222:29 14 (map1 _)
222:29 13 (map1 _)
222:29 12 (map1 _)
222:29 11 (map1 _)
222:29 10 (map1 _)
222:29 9 (map1 _)
222:29 8 (map1 _)
222:17 7 (map1 ("/var/www/gallery/albums//2010jan…" …))
In unknown file:
6 (_ # …)
In ice-9/eval.scm:
293:34 5 (_ #(#(#(# …) #) #))
293:34 4 (_ #(#(#(#(#(#(#) …) …) …) …) …))
293:34 3 (_ #(#(#(#(#(#(#) …) …) …) …) …))
In ice-9/popen.scm:
133:22 2 (open-pipe* "r" _ . _)
112:20 1 (open-process "r" "/bin/sh" "-c" "LC_ALL=C jhead -q /va…")
In unknown file:
0 (pipe)

ERROR: In procedure pipe:
In procedure pipe: Too many open files
```