rpc: add 'rpc' as always-available module

[meowsbits]

The MetadataAPI module 'rpc' is registered on
the creation of a new handler.

Signed-off-by: meows b5c6@protonmail.com

---

Fixes

A incorrect error ERROR[03-16|07:48:24.157] Unavailable modules in HTTP API list unavailable=[rpc] available="[admin debug web3 eth txpool personal ethash miner net]" is logged if a user configures --rpcapi=rpc[|,...]

./build/bin/geth --datadir=/tmp/gethddd --nodiscover --maxpeers=0 --rpc --rpcapi=admin,debug,eth,ethash,miner,net,personal,rpc,txpool,web3

07e1ff053b63af9b75c446c1a6aa7dbed878727ad2052b376648d46d@127.0.0.1:30303?discport=0"
INFO [03-16|07:48:24.157] IPC endpoint opened                      url=/tmp/gethddd/geth.ipc
ERROR[03-16|07:48:24.157] Unavailable modules in HTTP API list     unavailable=[rpc] available="[admin debug web3 eth txpool personal ethash miner net]"
INFO [03-16|07:48:24.158] HTTP endpoint opened                     url=http://127.0.0.1:8545 cors= vhosts=localhost

[holiman]

Since its always available, why would a user specify it explicitly?
And if we do add support for adding it explicitly, shouldn't the behaviour when not explicitly adding be to also not expose it?
I think we should close this PR.

[karalabe]

The rpc namespace is a bit of a special snowflake. There's no real way to make it behave exactly like the others since it's a built in metadata endpoint. I think we should just tell people that it's always available and not specify it. Simpler than adding yet more code to work around a nicety.

[meowsbits]

imo at least it shouldn't print an error if the user does configure it