sftp: Always use port 22 if not specified

Currently, the default port - specified in mount file - is removed from uri on client side, but it is not used by backend explicitely. It is not a problem until the default port is changed in sshd_config and somebody wants to connect to the server using the default port from mount file. Let's use the port 22 explicitely, similarly as other backends do (i.e. ftp, afc, afp).

Closes: #129