Add a copy file routine

Add io_file_cp() which can be used to copy an existing source file to
another location. We will need this for our new hash-based note file
names.

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

src/calcurse.h

@@ -693,6 +693,7 @@ void             io_set_lock (void);
 unsigned         io_dump_pid (char *);
 unsigned         io_get_pid (char *);
 int              io_file_is_empty (char *);
+int              io_file_cp (const char *, const char *);
 
 /* keys.c */
 void      keys_init (void);