Added networking support for Strife conversations.

This adds client-server netcode for conversations.

This also backports two commits from upstream GZDoom:

• Fix player not being able to talk to a Strife NPC during netplay, if another player talked to that NPC first.
• Made Strife NPC's dialog voice audible for all players.

While not essential, they're small and improve the multiplayer experience enough I thought it was worth it to add them. Fortunately, the only significant change to the conversation code since GZDoom 1.8.6 seems to be the ZScript-ification of the conversation menu, which neither commit touches.

With this it should be possible to play Strife online now -- although, nothing else was done to actually better the co-op experience, just that conversations function now.