revision: solve the implicit user name problem of

revision: solve the implicit user name problem of
          external-kex and gssapi
          switch back to old username if both fail
auth2.c sshconnect2.c and gss-serv.c are modified
auth2.c defines two new variables to hold the old username
        and recover to old username when both fail
sshconnect2.c send empty usernames to server for
        external-kex and gssapi with implicit username option
gss-serv.c check the username again in the verification