Forward connections without username/password to next hook in chain (#396)
Forward connections without username/password to next hook in chain (#396)

`vmq_passwd` rejected connections without username/password with `error` which
prevents further handling in later hooks in chain. This is now changed to
forward the event to next hook by returning `next` for connections without
username/password.

erlio/vernemq,erlio/vernemq,erlio/vernemq,erlio/vernemq