parallel port: initialize array to NULL to be sure
ISA PnP emulation: clarify some fields. use host_write* macros to build
structure. add code to prevent OS from allocating the READ_DATA port
below 0x200 (PnP standard says only 0x200-0x3FF is permissable). Add
code to return 0 from BIOS PnP interface if no PnP devices are assigned
(as specified in the standard).