Revised error handling

- Functions warn() and die() replaced by GNU-like error(3) function
- Register cleanup() with atexit(3)
- Functions called by cleanup() are marked with CLEANUP and are not allowed to
  call exit(3)

types.h

@@ -21,6 +21,8 @@
 
 #include <stdbool.h>
 
+#define CLEANUP
+
 typedef enum {
 	BO_BIG_ENDIAN,
 	BO_LITTLE_ENDIAN