cmd/geth: add 'dumpgenesis' cmd

[meowsbits]

Adds geth dumpgenesis command, which writes the configured genesis in JSON format to stdout.

This provides a way to generate the data (structure and content) that can then be used with the geth init command.

$ ./build/bin/geth dumpgenesis | jj -p | head -30
{
  "config": {
    "chainId": 1,
    "homesteadBlock": 1150000,
    "daoForkBlock": 1920000,
    "daoForkSupport": true,
    "eip150Block": 2463000,
    "eip150Hash": "0x2086799aeebeae135c246c65021c82b4e15a2c451340993aacfd2751886514f0",
    "eip155Block": 2675000,
    "eip158Block": 2675000,
    "byzantiumBlock": 4370000,
    "constantinopleBlock": 7280000,
    "petersburgBlock": 7280000,
    "ethash": {}
  },
  "nonce": "0x42",
  "timestamp": "0x0",
  "extraData": "0x11bbe8db4e347b4e8c937c1c8370e4b5ed33adb3db69cbdb7a38e1e50b1b82fa",
  "gasLimit": "0x1388",
  "difficulty": "0x400000000",
  "mixHash": "0x0000000000000000000000000000000000000000000000000000000000000000",
  "coinbase": "0x0000000000000000000000000000000000000000",
  "alloc": {
    "000d836201318ec6899a67540690382780743280": {
      "balance": "0xad78ebc5ac6200000"
    },
    "001762430ea9c3a26e5749afdb70da5f78ddbb8c": {
      "balance": "0xad78ebc5ac6200000"
    },
    "001d14804b399c6ef80e64576f657660804fec0b": {

[fjl]

Need to check:

geth dumpgenesis
geth dumpgenesis --rinkeby
geth dumpgenesis --datadir ~/my-data-dir 

[holiman]

LGTM.
These works as expected: build/bin/geth --testnet dumpgenesis, build/bin/geth --goerli dumpgenesis, build/bin/geth dumpgenesis

[cryptohazard]

Question: shouldn't this function also dump a custom genesis?