kernel - move proc->p_lock, add lwp ucred caching

kernel - move proc->p_lock, add lwp ucred caching

* Move p_lock out from the struct proc copy area, change it from a
  char to an int, and make PHOLD/PRELE MPSAFE.

* System calls now cache a read-only copy of p->p_ucred in
  lp->lwp_syscall_ucred.  This will be used by system call code
  to access the ucred in a MPSAFE manner.