network: merge link_foreignize_config() and link_drop_foreign_config()

network: merge link_foreignize_config() and link_drop_foreign_config()

When a reconfiguration of an interface is triggered, previously we
call link_foreignize_config(), which sets all static configurations as
foreign, then later call link_drop_foreign_config(), which drops
unnecessary foreign configurations.

This commit merges these two steps into one, link_drop_unmanaged_config(),
which drops unnecessary static and foreign configurations.

Also, this renames link_drop_managed_configs() to
link_drop_static_config(), as it only drops static configurations.
Note that dynamically aquired configurations are dropped by
link_stop_engines().