Allow free selection of MTU by the user

Allow free selection of MTU by the user

Various ISPs (or equipment?) seem to hand out an MTU of only 576 bytes.
Hwoever, this does not seem to be intentional which is why we would like
to manually overwrite this in the configuration.

dhcpcd only allows setting a maximum MTU of 1472 bytes which does not
seem to have any rationale (any more). Although Ethernet might limit any
MTU to less, IPv6 and IPv4 support MTUs of up to 64KiB.

This patch allows the user to configure the MTU freely with providing
some sanity check.

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