Opens a CHM helpfile quickly.
If the file opened is a valid CHM helpfile, only essential headers will be read. A mschmd_header structure will be still be returned, as with open(), but the mschmd_header::files field will be NULL. No files details will be automatically read. The fast_find() method must be used to obtain file details.
In the case of an error occuring, NULL is returned and the error code is available from last_error().
The filename pointer should be considered "in use" until close() is called on the CHM helpfile.