Fixes for the %U and %G problems people have reported.

Fixes for the %U and %G problems people have reported.
Essentially, multiple session_setup_and_X's may be done
to an smbd. As there is only one global variable containing
the requested connection name (sessionsetup_user), then any
subsequent sessionsetups overwrite this name (causing %U
and %G to get the wrong name). This is particularly common
when an NT client does a null session setup to get a
browse list after the user has connected, but before
a share has been mounted.

These changes store the requested_name in the vuid structure
(so this only really works for user level and above security)
and copies this name back into the global variable before
the standard_sub call.

Jeremy.