swarm: multiple --ens-api flags

Allow multiple --ens-api flags to be specified with value format
[tld:][contract-addr@]url.

Backward compatibility with only one --ens-api flag and --ens-addr
flag is preserved and conflict cases are handled:

 - multiple --ens-api with --ens-addr returns an error

 - single --ens-api with contract address and --ens-addr with
   different contract address returns an error

Previously implemented --ens-endpoint is removed. Its functionality
is replaced with multiple --ens-api flags.