md4c.h: Rename and refactorize MD_RENDERER struct. * Rename MD_RENDERER to MD_PARSER. (Typedef to provide the original name is provided to minimize disruption of existing code.) * Reorder its members to make better sense. * Add abi_version member (hopefully, it shall never be needed ;-) * Update md2html utility to deal with the change. Rationale: This is done in order to prepare for long-term maintenance of ABI compatibility, as there is no work-in-progress to be buildable as shared lib, and be included in some Linux distros.