Defer the program termination on error until all files are closed and
(if applicable) all incompletely-written output files are deleted.
In addition, perform the following maintenance tasks:
* Rename and document the internal helpers used by the functions
`png2pnm` and `pnm2png`.
* Unset the executable permission bits for the *.bat test programs.
Raise the minimum required CMake version to 3.5.
Add the configuration option `PNGMINUS_USE_SYSTEM_PNG` for compiling
and linking with the system libpng library instead of the internal one.
Remove the old configuration option `PNGMINUS_USE_STATIC_LIBRARIES`.
When using the internal libpng (via `PNGMINUS_USE_SYSTEM_PNG=OFF`),
simply enforce static linking and produce single-file executables.
Rename the scripts "png2pnm.sh" (etc.) to "test_png2pnm.sh" (etc.),
to make it obvious that they are test drivers, not program launchers.
Add a .gitignore file for project-specific build and test artifacts.
