pay attention to EMSGSIZE errors when sending pmtu verification packets
and drop the pmtu accordingly
This way, when we actually *do* get pmtu info from the network/kernel
we can discover the correct pmtu much more quickly. Especially in the
absence of other traffic, forwarding of which was previously the only
place where we'd check for EMSGSIZE.

Fixes #12
drop maxpayload in conjunction with clientPMTU
Seems odd to do the latter w/o the former. And wrong.
8 is better than 10
It's quite common for the real pmtu to be 8 less than what was
advertised.
tiny refactor: rename, move