Considering that pnglibconf is a whole mini-system for configuring and
building libpng, it should be placed in its own dedicated subdirectory.
Any future additions, including tests and documentation, should go into
this subdirectory.
One exception: pnglibconf.h.prebuilt is moved to the top-level source
directory, where pnglibconf.h would be. This follows the organization
pattern typically used with the "*.in" files, for example.
All references to the pnglibconf artifacts, found in machine-readable
code and in human-readable comments, are updated accordingly.
Remove all makefiles that used to serve specific Unix and Unix-ish
operating systems. We still retain the generic Unix makefile.std,
as well as the compiler-specific (Unix and non-Unix) makefiles and
scripts.
Update scripts/README.md and manuals/libpng-install.txt accordingly.
Rename ANNOUNCE, AUTHORS, LICENSE, README, TODO, TRADEMARK to
ANNOUNCE.md, AUTHORS.md, LICENSE.md, README.md, TODO.md, TRADEMARK.md.
Also rename INSTALL to INSTALL.txt, to be converted later to INSTALL.md.
