bug in panel

[Tom Tromey <tromey creche cygnus com>]

The function panel_session_save seems to assume that it will only be
called when a shutdown is being requested.

However, this is not the case.  A session can be checkpointed (saved
but not terminated).  In this case, panel_session_save will be called,
but the program will be expected not to exit.

Part of the function should check the `is_shutdown' argument before
destroying widgets and such.

Tom