WIN32 versions of io functions

src/io.h

@@ -88,10 +88,10 @@ int File_length(const char *fname);
 int File_length_file(FILE * file);
 
 /// Returns true if a file passed as a parameter exists in the current directory.
-int File_exists(char * fname);
+int File_exists(const char * fname);
 
 /// Returns true if a directory passed as a parameter exists in the current directory.
-int  Directory_exists(char * directory);
+int  Directory_exists(const char * directory);
 
 /// Check if a file or directory is hidden. Full name (with directories) is optional.
 int File_is_hidden(const char *fname, const char *full_name);