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, 1 insertions, 0 deletions

diff --git a/util.h b/util.h
index 6115dcf..8053750 100644
--- a/util.h
+++ b/util.h
@@ -64,6 +64,7 @@ int uri_hasscheme(const char *);
 int uri_makeabs(struct uri *, struct uri *, struct uri *);
 int uri_parse(const char *, struct uri *);
 
+void checkfileerror(FILE *, const char *, int);
 void parseline(char *, char *[FieldLast]);
 void printutf8pad(FILE *, const char *, size_t, int);
 int  strtotime(const char *, time_t *);