discover: Track both configured and current autoboot settings

discover: Track both configured and current autoboot settings

If autoboot is enabled but later disabled or cancelled by, for example,
an IPMI override then the nc-config screen will set the autoboot widget
as disabled. If the user then makes and saves a change in nc-config,
autoboot will also be saved as disabled. This accidental change is
particularly awkward if the user is attempting to remove an IPMI
override.

Instead only ever change the autoboot setting if the user explicitly
changes it. Use a new helper function 'config_autoboot_active()' to
determine the current autoboot status where needed.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>