kernel - Make adjustments to the per-lwp ucred

kernel - Make adjustments to the per-lwp ucred

* Rename lwp_syscall_ucred to lwp_ucred and move ucred synchronization
  from syscall2() to userenter().  It now runs on any user entry into
  the kernel, not just for system calls.

  NOTE: A VM fault from the kernel itself does not call userenter() and
        does not trigger another synchronization.  This is the behavior
	we desire, to ensure that lwp_ucred is stable.

  There are numerous non-syscall cases (e.g. signal, fault, etc) where
  we may need to access a safe ucred.

* Remove the SMP conditional.  lwp_ucred is now tracked for both UP and SMP.
  There is no point optimizing this for UP since there is basically no cost
  for tracking lwp_ucred.