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)

commands.c

@@ -30,7 +30,6 @@
 #define _IMAGE_CONFIG
 #include "config.h"
 
-void cleanup(void);
 void remove_file(int, bool);
 void load_image(int);
 void open_info(void);
@@ -64,7 +63,6 @@ bool cg_quit(arg_t _)
 				printf("%s\n", files[i].name);
 		}
 	}
-	cleanup();
 	exit(EXIT_SUCCESS);
 }