ENet by default uses 1400 as enet packet fragment max size, adding ICMP and
IPv4 headers this may exceed the MTU for some VPN solutions [1], so provide
a lower default.
MTU negotiation server-side needs [2], which was merged after enet-1.3.17,
so the user configured value may be ignored on older versions.
[1] https://github.com/lsalzman/enet/issues/132
[2] https://github.com/lsalzman/enet/pull/222