Migrate rlibmap to rootcling

Have rootcling generate rootmap files. When passed "--rootmap --libname lib/libout.lib" it will generate lib/libout.rootmap.

Rationale: rootmap files must now contain template (forward) declarations. Thus we need rlibmap to "understand" the types in LinkDef files.

The best way out (also for a future implementation of autoloading of functions etc) it to migrate rlibmap to become rootcling --rootmap. Similar functionality should already exist for the genreflex mode anyway.