liblzma: Rename the private API header lzma/lzma.h to lzma/lzma12.h.

It can be confusing that two header files have the same name.
The public API file is still lzma.h.
Docs: Update comments to refer to lzma/lzma12.h in example programs.
xz: Fix uint64_t vs. size_t which broke 32-bit build.

Thanks to Christian Hesse.