chg: dev: Don't open route socket if we don't need it

When automatic-interface-scan is disabled, the route socket was still
being opened.  Add new API to connect / disconnect from the route socket
only as needed.

Additionally, move the block that disables periodic interface rescans to
a place where it actually have access to the configuration values.
Previously, the values were being checked before the configuration was
loaded.

Closes #4757

Merge branch '4757-dont-open-routing-socket-if-not-needed' into 'main'

Closes #4757

See merge request isc-projects/bind9!9122