network: make 'networkctl reconfigure' work safely even when KeepConfiguration=dhcp...

network: make 'networkctl reconfigure' work safely even when KeepConfiguration=dhcp or yes

Previously, even if KeepConfiguration=dhcp or yes is specified in the
new .network file, dynamic configurations like DHCP address and routes
were dropped when 'networkctl reconfigure INTERFACE' is invoked.

If the setting is specified, let's gracefully handle the dynamic
configurations. Then, 'networkctl reconfigure' can be also used for
an interface that has critical connections.