luci-mod-network: handle dynamic device configuration defaults

Most uci network device configuration settings have no specific default
value which means that netifd will only alter the related sysfs parameters
if an explicit value is specified in uci. When omitted from the
configuration, the related sysfs setting is left untouched.

This behaviour collides with LuCI's approach of purging boolean options
from the configuration if they match their default value, leading to the
inability to disable or enable certain settings as described in #6219.

Solve this issue by replacing flag widgets with tri-state selects offering
an "automatic" (default) as well as an explicit "enabled" and "disabled"
choice. Also query sysfs on load and preset the automatic choice with an
indication whether the underlying sysfs settings is currently active or not.

Fixes: #6219
Signed-off-by: Jo-Philipp Wich <jo@mein.io>