Add a `DefaultDatabaseFile` configuration element.

One could, for example, set this to `/home/foo/.sdb.db` to have
SDB save the debugger database there by default without having
to specify the file name whenever the `db` command is invoked.
Add `SaveDatabaseAutomatically` / `LoadDatabaseAutomatically` configuration elements.

If `SaveDatabaseAutomatically` is set to `true` and the
`DefaultDatabaseFile` element is set to a non-empty string,
SDB will save the debugger database to that file on exit.
Similarly, `LoadDatabaseAutomatically` will load it on startup.