Add usnjrnl.h (copy from kernel driver):

- describe the $UsnJrnl on disk structures

Finish logfile.h: in particular, add:

- more about the $LogFile on disk structures

Remove ifdef HAVE_CONFIG_H and make config.h compulsory.  Further, place
config.h in include/ntfs/config.h at least when distributing (make install,
make dist, make rpm).