kernel - use new td_ucred in numerous places

kernel - use new td_ucred in numerous places

* Use curthread->td_ucred in numerous places, primarily system calls,
  where curproc->p_ucred was used before.

* Clean up local variable use related to the above.

* Adjust several places where p_ucred is replaced to properly deal
  with lwp threading races to avoid accessing and freeing a potentially
  stale ucred.

* Adjust static procedures in the ktrace code to generally take lwp
  pointers instead of proc pointers.