cmd/geth: added --mainnet flag

[MariusVanDerWijden]

closes #21795

TODO:

• check that genesis is okay

[MariusVanDerWijden]

@holiman I remembered in the discussion you said something about checking that the genesis is okay.
But I don't know how to do it, any hints?

[holiman]

Sorry, I missed that you had asked me something. So, I'm not 100% sure you need to do anything in particular. But check that "odd stuff" doesn't happen e.g when you do

geth --goerli --datadir /tmp/foo init
geth --mainnet --datadir /tmp/foo 

or

geth --goerli
ctrl-c
geth --mainnet --datadir=~/.ethereum/geth/goerli

Also, maybe we should check the result of doing:

geth --mainnet --networkid=1337

not sure what I would prefer, probably that the networkid=1337 "wins", but maybe it's better to exit with an error.

[MariusVanDerWijden]

So mainnet -> rinkeby produces an error. However init with rinkeby then mainnet does not error (which it should)

• edit works now

[holiman]

LGTM