stricter error checking in file streams (input, output)

This also makes the programs exit with a non-zero status when a read or write
error occurs.

This makes checking the exit status more reliable in scripts.

A simple example to simulate a disk with no space left:

	curl -s 'https://codemadness.org/atom.xml' | sfeed > f
	/mnt/test: write failed, file system is full
	echo $?
	0

Which now produces:

	curl -s 'https://codemadness.org/atom.xml' | sfeed > f
	/mnt/test: write failed, file system is full
	write error: <stdout>
	echo $?
	1

Tested with a small mfs on OpenBSD, fstab entry:

	swap /mnt/test mfs rw,nodev,nosuid,-s=1M 0 0

1 files changed, 3 insertions, 0 deletions

diff --git a/sfeed_opml_import.c b/sfeed_opml_import.c
index 0844b5c..6c0bd0e 100644
--- a/sfeed_opml_import.c
+++ b/sfeed_opml_import.c
@@ -101,5 +101,8 @@ main(void)
 	xml_parse(&parser);
 	fputs("}\n", stdout);
 
+	checkfileerror(stdin, "<stdin>", 'r');
+	checkfileerror(stdout, "<stdout>", 'w');
+
 	return 0;
 }