Purpose
Creates a t_FileInfo structure to describe the given FileName. This routine should only be used if you are going to add the given FileName to the given lq-text Database db; to get a FileInfo describing a file already in the index, use LQT_NameToFID and LQT_FIDToFileInfo.
Returns
If the file is not already in the database, a new FID is allocated, and a newly malloc'd t_FileInfo object is returned, complete with a stdio FILE pointer already opened, either as a file or as a pipe, depending on the file type and filter table; it is the caller's responsibility to call LQT_DestroyFileInfo to free the memory and close the stdio stream.
On error, or if the file is already in the database and has not changed since it was last indexed, a warning is issued and a NULL pointer is returned.