CV64: Some DeathLink adjustments

[LiquidCat64]

What is this fixing or adding?

Some DeathLink-related things, mainly:

• When a DeathLink was sent by a CV64 player, other players that were connected to that same CV64 slot would not receive it. This PR fixes that.
• Sent death messages now include the in-game area the death occurred at, much like CotM's implementation.
• Added handling for when another game includes a death cause message that is just a blank string, i.e. Super Mario World. There should be no more blank in-game death message textboxes.
• Stopped the client from spam-sending goal packets after goal completion.

How was this tested?

Connected two separate instances of the same CV64 slot to the same localhosted server (connected with their own BizHawk Client instances), got the player killed in one instance, and made sure the other player got killed as well while the first one did not get killed again upon respawning. Also got the player killed in most maps to make sure the area names in the death messages worked.

[Exempt-Medic]

Hmm, an empty cause goes against spec and worlds should not be doing that

[LiquidCat64]

SMW's client.py calls SNIClient.py's handle_deathlink_state() with no parameter for the death_text variable, and the default value for death_text there is "", so the result is an empty cause message, which in turn, results in an empty textbox in CV64 that looks weird. I think I've had at least one bug report from a CV64 player in the past playing DeathLink with SMW who thought the game had bugged?

[LiquidCat64]

Bonus fix: the client should no longer be spam-sending goal packets after goal completion. This was a fix I had to make to CotM while it was in review.