Allow core dumping via a fifo

Solaris doesn't mind if ./core is a fifo; it happily dumps core through the fifo. (Or so I've heard.)
Here's a patch to 2.4.9 that gives Linux this same ability. To test it, try
mkfifo core
cat core > core2 &
cat
(hit ^\ to force a core dump in 2nd cat)
You should have a usable core file in 'core2'.

This is only my second patch ever, so be kind... I'm sure there's something wrong with it, somewhere, but it did seem to work in my quick tests.

This is also a preliminary step towards what I described in my earlier post today ("remote core dumping").
- Dan

@@ -954,7 +954,7 @@
 	if (d_unhashed(file->f_dentry))
 		goto close_fail;
 
-	if (!S_ISREG(inode->i_mode))
+	if (!S_ISREG(inode->i_mode) && !S_ISFIFO(inode->i_mode))
 		goto close_fail;
 	if (!file->f_op)
 		goto close_fail;