add configuration option where, if machine=cga or machine=hercules, the
640kb conventional memory area could be extended out to just below
CGA/Hercules video RAM (736KB CGA, 704KB Hercules).
VGA bugfix: when changing memory map, instead of "resetting" pages
(which sets them back to RAM) change them instead to "unmapped" where
reads return 0xFF and writes are ignored (like real hardware).
vga map reset updated to follow "adapter rom is ram" setting.