dhcpcd: Force setting MTU through dhcpcd

dhcpcd: Force setting MTU through dhcpcd

The DHCP server can instruct clients to configure a certain MTU.

This used to be done by setting the MTU of the interface. However,
dhcpcd has changed this behaviour using routes to.

We used to have a modified version of the old mechanism which no longer
works well with the new system and is therefore to be dropped.

This is the first commit in the series implementing the new behaviour
and telling dhcpcd to use the configured MTU.

Fixes: #12563
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>