Fix crash with weird filetimes (and cleanup the code)

Bug: #1579472 Runtime Error during folder compare
https://sourceforge.net/tracker/index.php?func=detail&aid=1579472&group_id=13216&atid=113216

I couldn't figure out to properly handle weird
filetimes got from findfile. So I decided to use stat()
instead as it returns missing filetimes as zeros. Or so
I thought. Luckily it returns them as negative value,
so we can at least detect them.

While I changed FileInfo to use stat() I could just
remove the code calling findfile() and cleanup the
code. Then I noticed that FileInfo's Update() and
DiffFileInfo's Update() became identical other than
return value. So I added return value for FileInfo's
Update() and removed the DiffFileInfo's update.

I also did similar findfile() to stat() change for
dirscan.cpp's enumeration routine.

I also added some checks for filetimes being zeroes.