Warn when reloading with unsaved modifications

Since the reload operation overwrites all changes, warn before reloading
if there are unsaved modifications.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/calcurse.h

@@ -779,6 +779,9 @@ unsigned io_dump_pid(char *);
 unsigned io_get_pid(char *);
 int io_file_is_empty(char *);
 int io_file_cp(const char *, const char *);
+void io_unset_modified(void);
+void io_set_modified(void);
+int io_get_modified(void);
 
 /* keys.c */
 void keys_init(void);