Add option to redirect luxi socket for debugging

Currently, the choice between masterd and conf/queryd is made
statically, based on whether a query is converted or not and whether
split queries are enabled. This makes it hard to test and develop
these new queries, so this patch adds an override for the socket.

The choice for going with two hardcoded strings and arbitrary socket
fallback is done so that it's easy to use this in virtual clusters as
well.

Additionally, the patch improves the error message on failed connects
when we use arbitrary sockets; before, it always "blamed" confd for
non-master sockets.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>