copy: move sync_rights() to copy.c and rename copy_rights()

It's so similar to copy_access(), hence let's move it over and rename it
in similar style to the rest of the functions.

No change in behaviour, just moving things over.

src/basic/fileio.h

@@ -118,6 +118,4 @@ int safe_fgetc(FILE *f, char *ret);
 
 int warn_file_is_world_accessible(const char *filename, struct stat *st, const char *unit, unsigned line);
 
-int sync_rights(int from, int to);
-
 int rename_and_apply_smack_floor_label(const char *temp_path, const char *dest_path);