network: dhcp4: also support semi-static routes with Gateway=_dhcp4 when UseGateway...

network: dhcp4: also support semi-static routes with Gateway=_dhcp4 when UseGateway=no or UseRoutes=no

This makes the default gateway is read from classless static routes or
router option even if UseGateway=no or UseRoutes=no, and will be used
when configuring semi-static routes such that specified with Gateway=_dhcp4.

This also changes the behavior of RoutesToDNS= or RoutesToNTP=.
Previously, the DNS or NTP servers are not in the same network, then the
routes to the servers were not configured when UseGateway=no or
UseRoutes=no. With this commit, the default gateway in classless static
routes or router option will used to connecting the servers even if
UseGateway=no or UseRoutes=no.

Fixes #20208.