- convert {set, fill}_??regs() and ptrace_single_step() API to taking

- convert {set, fill}_??regs() and ptrace_single_step() API to taking
  struct lwp instead of struct proc, change their caller accordingly
- in i386/include/md_var.h, add forward declaration of struct lwp and eliminate
  the no longer needed forward declaration of struct proc
- in i386/include/reg.h, add forward declaration of struct lwp and struct proc
- minor style and white space changes

Requested-and-Reviewed-by: corecode