First patch that prepares for the adding of multi-firmware

First patch that prepares for the adding of multi-firmware
support. Changes are rather simplifications of the SCRIPTS
interface (prior to complexifying it again;) ), dead code
removes and comment fixes.

Code removed:
- Handling of kernel variables referenced from SCRIPTS.
- Handling of selection without ATN.

Slightly rewritten:
- Handling of illegal phase (4/5) and data overrun conditions.

Simplifications:
- Extended error flag and bits now only set from the C code.
- Move the extended error status (xerr_status) and nego
  status (nego_status) outside the data structure accessed
  by SCRIPTS (struct dsb).
- Get rid of the script status field (scr_st).
- Only patch SCR_NO_OP SCRIPTS instructions to adapt SCRIPTS
  to actual chip capabilities.

Cosmetic changes:
- Miscellaneous comments in SCRIPTS.
- FreeBSD_4_Bus define replaced by FreeBSD_Bus_Io_Abstraction.