networkd,udev: clean up MTU handling

networkd,udev: clean up MTU handling

This cleans up handling of MTU values across the codebase. Previously
MTU values where stored sometimes in uint32_t, sometimes in uint16_t,
sometimes unsigned and sometimes in size_t. This now unifies this to
uint32_t across the codebase, as that's what netlink spits out, and what
the majority was already using.

Also, all MTU parameters are now parsed with config_parse_mtu() and
config_parse_ipv6_mtu() is dropped as it is now unneeded.

(Note there is one exception for the MTU typing: in the DCHPv4 code we
continue to process the MTU as uint16_t value, as it is encoded like
that in the protocol, and it's probably better stay close to the
protocol there.)