Make setting up a regtest network easier

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..