concat: Use plain read/write instead of buffered stdio

If we are just copying data from one file to another, we don't need to
fill a complete buffer, just read a chunk at a time, and write it to the
output.

util.h

@@ -64,6 +64,7 @@ int eregcomp(regex_t *, const char *, int);
 
 /* io */
 ssize_t writeall(int, const void *, size_t);
+int concat(int, const char *, int, const char *);
 
 /* misc */
 void enmasse(int, char **, int (*)(const char *, const char *, int));