OpenSystemsDevelopment / qpdf

This change updates all occurrences of `std::endl` to `\n` across various files for improved efficiency and consistency. It reduces unnecessary flushing of output buffers associated with `std::endl` while maintaining the same output format.

Run this:

for i in  **/*.cc **/*.c **/*.h **/*.hh; do
  clang-format < $i >| $i.new && mv $i.new $i
done

Turns out unreadCh is much more efficient than seek(-1, SEEK_CUR).
Update comments and code to reflect this.

This is all that ever worked. The test suite was trying to do
something different from ClosedFileInputSource.

During periods of intensive operation on a specific file, this method
can reduce the overhead of repeated open/close operations.

ClosedFileInputSource is an input source that keeps the file closed
when not reading it.