* New settings option for a custom SSH host (#3763)

* let default ssh listen addr empty

- Update import paths from github.com/go-gitea to code.gitea.io
- Fix import path for travis

See https://docs.travis-ci.com/user/languages/go#Go-Import-Path

When use builtin SSH server with concurrent operations, there are probabilities
One connection could use the command from another connections.

Fix this by set SSH_ORIGINAL_COMMAND for each command, not set in global scope.

According to the docs [1], the Reply method must be called for all
requests where WantReply is true. This fixes a hanging java ssh
implementation (jsch) which sets WantReply flag and waits for reply from
the server.

[1] https://godoc.org/golang.org/x/crypto/ssh#Request.Reply