Doxygen: Refactor Doxyfile.in to doxygen/Doxyfile.

Doxygen: Refactor Doxyfile.in to doxygen/Doxyfile.

Instead of having Doxyfile.in configured by Autoconf, the Doxyfile
can have the tags that need to be configured piped into the doxygen
command through stdin with the overrides after Doxyfile's contents.

Going forward, the documentation should be generated in two different
modes: liblzma or internal.

liblzma is useful for most users. It is the documentation for just
the liblzma API header files. This is the default.

internal is for people who want to understand how xz and liblzma work.
It might be useful for people who want to contribute to the project.