pdfseparate: improve the path building

Make use of snprintf to limit the output to the pathName buffer;
while I'm there, expand its size to 4096 (might help longer paths),
although a better fix would be dynamically allocate its length
(and/or using GooString, maybe).
Windows: Fix CreateFile fails with ERROR_SHARING_VIOLATION

Bug #69597
pdftocairo: check file opening failure in beginDocument()
Update copyrights
0.24.2