[krbdev.mit.edu #6340] mmap for kdb update log

[Ken Raeburn via RT]

Just noting for the future: We should be more paranoid about the  
handling of the KDB update log file.  The use of mmap makes me a bit  
nervous -- what if the page isn't there and can't be allocated?

I did change the update log creation not to create sparse files on  
some operating systems.  However, I think it may be possible in the  
current code for the file setup to fail if the file system is full at  
setup time, but leave the file in a state where a later invocation  
might assume it to be ready to use.

Granted, if your KDC's disk fills, you've got other things to worry  
about, but we don't need the KDC crashing and leaving incomplete files  
around to add to it.  A clean exit, and more importantly leaving the  
update log file in a state where the next invocation will still  
recognize the need to allocate storage, would be an improvement.   
Using fread instead of mmap would help us trap the errors at run time  
instead of crashing, too, if some case gets missed.

(Alternatively: If we don't use mmap, we could forego the initial file  
setup and just allocate the space as we need it.)

Ken