cmd/bzzd: swarm daemon fixes (#3359)

* cmd/bzzd: add missing p2p/discovery flags
* cmd/bzzd: fix two bugs crashing bzzd if bootnodes flag given
* cmd/bzzd: make no swap default, renamed flag bzznoswap->bzzswap
* internal/web3ext: correct methods for bzz IPC module
* cmd/bzzd: ethapi param not mandatory. Warning if no blockchain
* cmd/bzzd: correct default IPC modules in help string
* cmd/utils: fix help description for networkId - add Ropsten
* cmd/bzz, swarm/api, swarm/network: add swarm networkId flag
* cmd/bzzd: change nosync flag to sync and BootTFlag