Implement Marcin Bulandra's suggestion of only automatically updating the

Implement Marcin Bulandra's suggestion of only automatically updating the
port number in the GUI when the connection type is changed if the current
port number is the standard one for the current protocol.
It's not perfect, but it should make the common case of tabbing through the
Session panel easier when starting non-SSH connections on odd ports.

[originally from svn r7635]