New option: EchoCfg->Toggles->Fwd Circular Msgs to Links

(or by adding NOCIRCULARFWD to your sbbsecho.cfg):
When "Circular Path Detection" is enabled (the default), the old behavior was
to still forward the message to any links (if there are any), but not import
the circular/looped message (likely a dupe). Now, if you set this new option
to no (or add "NOCIRCULARFWD" to your sbbsecho.cfg), the message will be
completely ignored and not fowarded to any links (if there are any).
If you have no downlinks or you have circular path detection disabled, this
new option does nothing. The default is the old behavior (forward circular
messages).
Also updated the Toggle Options help text in echocfg (many options weren't
documented.