- Streamline the usage of sector_t over the strategy routines in question a
   bit. Streamline the do_request code in ide-disk.c.

- Improve the readability of start_request in ide.c.

- Remove obsolete/nowhere used stuff from hdreg.h.

- Splitup special_t into classical flag field.

- Use only a single field to determine the capacity of a drive.  Make this
   field and the code paths it follows as far as possible use the sector_t
   instead of a hard coded integer types.  This increases the chances that at
   some distant point in time we will indeed be able to use 64 bit wide sector_t
   entities. (Disks are getting huge those times now...)