Make setting up a regtest network easier

[stevenroose]

Some things I have ran into many times in the past that are just really annoying for developer experience:

• -regtest gives you bitcoin regtest. Should really make that equivalent to -chain=elementsregtest.
• -server=1 doesn't print what RPC port is being listened to. This might be a bitcoind issue too, but anyone can google the default regtest RPC port for Bitcoin, I always spend 3 minutes trying to figure out what the one for elementsregtest is and eventually giving up and using -rpcport=8888..
• There are no coins in the default chain. I know this might be a feature, but if no parent chain info is given, maybe there should be money. I tried adding -initialfreecoins=2100000000000000 -anyonecanspendaremine=1 also without success. So I'm now at figuring out how to add money. It seems like the rescanblockchain rpc is needed now.

All this which in Bitcoin is just bitcoind -regtest is a real pain for Elements..