Use !* which is better than -=
The LHS of -= may not exist in a list
radius_map2vp() returns 0 on not found, not -2

Because not finding a VP isn't an error.  It just means
that we can't do the requested action
Fix radius_map2request() to handle "head == NULL"

Which is now always valid.  The side-effect is that the operators
now *always* behave as defined, even if that means unexpected
behavior:

	Filter-Id := &Blah

if "Blah" attribute doesn't exist, Filter-Id is deleted.  That's
because you're setting it to the value of Blah.  Which is nothing.
Issue warning when we have invalid syntax

In the future, "User-Name !* ANY" will be required.
For now, "User-Name !* bob" will cause a warning message
to be printed in debugging mode.