network/dhcp4: create prefix route and route to gateway in the specified table with...

network/dhcp4: create prefix route and route to gateway in the specified table with Gateway=_dhcp4

Previously, the following setting
====
[Route]
Gateway=_dhcp4
Table=100
====
only configured the route in the specified table. But it was mostly
useless. This makes prefix route and route to the gateway are also
configured in the specified table.

Before:
====
$ ip route show table 100
default via 192.168.0.1 dev eth0 proto dhcp metric 1024
====

After:
====
$ ip route show table 100
default via 192.168.0.1 dev eth0 proto dhcp metric 1024
192.168.0.0/24 dev eth0 proto dhcp src 192.168.0.100 metric 1024
192.168.0.1 dev eth0 proto dhcp scope link src 192.168.0.100 metric 1024
====