OpenSystemsDevelopment / qpdf

Expose functions to the C API to create new loggers and to setLogger
and getLogger for QPDF and QPDFJob.

When there is no context for writing output or error messages, use the
default logger.

Run this:

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

This comment expands all tabs using an 8-character tab-width. You
should ignore this commit when using git blame or use git blame -w.

In the early days, I used to use tabs where possible for indentation,
since emacs did this automatically. In recent years, I have switched
to only using spaces, which means qpdf source code has been a mixture
of spaces and tabs. I have avoided cleaning this up because of not
wanting gratuitous whitespaces change to cloud the output of git
blame, but I changed my mind after discussing with users who view qpdf
source code in editors/IDEs that have other tab widths by default and
in light of the fact that I am planning to start applying automatic
code formatting soon.

* 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

Let argv be a null-terminated array. There is already code that
assumes this, and it makes it easier to construct the arguments.

This makes it much more convention to use the initializeFromArgv
functions since you can use string literals.