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, 10 insertions, 0 deletions

diff --git a/util.c b/util.c
index db43e0c..ed0b5c9 100644
--- a/util.c
+++ b/util.c
@@ -46,6 +46,16 @@ errx(int exitstatus, const char *fmt, ...)
 	exit(exitstatus);
 }
 
+/* Handle read or write errors for a FILE * stream */
+void
+checkfileerror(FILE *fp, const char *name, int mode)
+{
+	if (mode == 'r' && ferror(fp))
+		errx(1, "read error: %s", name);
+	else if (mode == 'w' && (fflush(fp) || ferror(fp)))
+		errx(1, "write error: %s", name);
+}
+
 /* strcasestr() included for portability */
 char *
 strcasestr(const char *h, const char *n)