Rework the 'guest account get's RID 501' code again...

Rework the 'guest account get's RID 501' code again...

This moves it right into the passdb subsystem, where we can do this in
just one (or 2) places.  Due to the fact that this code can be in a tight loop,
I've had to make 'guest account' a 'const' paramater, where % macros cannot be
used.  In any case, if the 'guest account' varies, we are in for some nasty
cases in the other code, so it's useful anyway.

Andrew Bartlett