Update validate function to return default target for zero value
confirmation targets, as indicated by the comment. This change
introduces a behaviour change for direct rpc calls to loopd (ie,
those not made by the loop cli tool). Previously, these calls
would fail and indicate that the conf target must be > 2, now
they will succeed with the default conf target. The loop cli
tool is unaffected because we already set the default value.
When clients upgrade from a previous version of loopd which did
not have this check, preexisting loops will be unaffected, because
loop ins had the default of 6 confirmations set, and loop outs with
<2 conf target would not have been created.
