Some more details, some rearrangement

SSH key exchange messages are often big enough to trigger MTU problems,
so the initial connection to the server will always work fine, but the
negotiation will never complete and SSH will hang.

In theory PMTU discovery should help to alleviate this problem (at least
to the extent of giving warnings in udp_write()), but it's often broken
by NAT gateways and other common nuisances.
Make the MTU suggestion slightly more useful

Since packets have nearly-constant overhead ("nearly" because of the
variable Ethernet frame header), we can suggest a more precise value.