Replace --[chain].[network] flags with --chain and --network

[mrfelton]

lnduses a --[chain].[network] flag to determine which chain/network is active.

lncli uses --chain and --network flags to do the same.

I think it might preferable if we follow the format of lncli rather than lnd since this tool is a cli for issuing one-shot commands, rather than spawning a persistent process, and I suspect that most users are more familiar with the lncli runtime options than the lnd startup flags.