- fixed the %U macro so that the old (and documented) semantics work

- fixed the %U macro so that the old (and documented) semantics work
again. This got broken with one of the substitute.c updates a couple
of months ago.

- also fixed %u to return the username from the current_user structure
  when called via a method that does not have direct access to the
  username. I cache the uidtoname() result to prevent thrashing nss.