Change the way RD=0 forwarded queries are handled.

Change the way RD=0 forwarded queries are handled.

Since forever, there has been special case code for forwarded queries
in the RD=0 case.  This special case code does a hardcoded RD=0
query to the specified forwarder.  This code has two consequences:

1. Even if the forwarder is marked recursive it gets a RD=0 query
2. The cache is not consulted at all

The corresponding unit tests actually test this behaviour, but after
historic digging with help from @rgacogne it turns out the the unit
test do not reflect the desired functionality, but the current state
of affairs to help with a refactoring PR.  That is good, since
refactoring should not change functionality.

But now the time has come to change the code to do the desired thing:

1. If an RD=0 query is received, do a cache only-lookup in all cases.
2. Never send a RD=0 query to a recursive forwarder

I already did a similar thing when I wrote the QName Minimization
code, introducing a conditional that only gets set for that case,
to avoid changing unrelated (to QM) functionality.