OpenSystemsDevelopment / qpdf

Add comments to force line breaks, parenthesize function arguments
that are contatenated strings, etc. -- these kinds of changes improve
clang-format's results and also cause emacs cc-mode to match
clang-format. After this type of change, most of the time, when
clang-format and emacs disagree, clang-format is better.

Configure emacs and clang-format 15 to the coding style I am choosing
for qpdf.

* Don't surprise people with deprecation warnings
* Provide detailed instructions and support for the transition

* Use unique_ptr in place of shared_ptr in some cases
* unique_ptr for arrays does not require a custom deleter
* use std::make_unique (c++14) where possible