dylanchu

no core logic was changed, just some formal parameter changes on specific functions for decoupling, like function `run` `runUDP` `handleConnection` `updatePasswd`. function `updatePasswd` was changed into method of type `PasswdManager`...

another consideration is functions in `util.go` should be small independent tools that don't rely on functions in the other three files. (but in fact still rely on global variables defined...