{set,fill}_{,fp,db}regs() fixup:
- Add dummy {set,fill}_dbregs() on architectures that don't have them.
- KSEfy the powerpc versions (struct proc -> struct thread).
- Some architectures had the prototypes in md_var.h, some in reg.h, and
some in both; for consistency, move them to reg.h on all platforms.
These functions aren't really MD (the implementation is MD, but the interface
is MI), so they should move to an MI header, but I haven't figured out which
one yet.
Run-tested on i386, build-tested on Alpha, untested on other platforms.
