Add new mport_setting_get and mport_setting_set functions to allow users to customize settings.  The sqlite3 master database now contains a settings table with name and val fields.

There are two obvious uses for this:

1. defining the country the user is in to pick the nearest mirror list.
2. storing the last time we fetched an index so it doesn't  try to do it all the time when it's out of date and we haven't generated one on stargazer yet.