Use the new ctrl_alloc_with_free to clean up a long-standing FIXME in

Use the new ctrl_alloc_with_free to clean up a long-standing FIXME in
the session saving code, in which the contents of the edit box giving
the current saved session name was stored in a horrid place with a
fixed length. Now it's dangling off sessionsaver_data as it always
ought to have been, and it's dynamically reallocated to the
appropriate length, and there's a free function that cleans it up at
the end of the dialog's lifetime.

[originally from svn r9923]