r17378: Fix the issues people have been having with mapped

r17378: Fix the issues people have been having with mapped
users (username map) and failure to connect to a
share. Essentially, even on a standalone system
we were going into the create_token_from_username()
code (I think by mistake) if the username was mapped.
Fixes bug #3991.

Volker & Jerry - please go over this with a very careful eye
and let me know if this isn't correct (I think it is,
but this isn't my code and it's a dangerous area for
me to be playing in :-).

Jeremy
(This used to be commit 0b5b2b53ec6e4c25b5f6645451dfce4aa7ae8a61)