connection backlog, avoid check_close() for channels that haven't had IO

auth requests). Regresssion in ff597bf2cfb0

EAGAIN but always saves a select() - needs testing with bandwidth-limited
and CPU-limited situations.

system clock changes

While at it, fix a few indentations and typo.

be more precise about maximum channel sizes

- Fix port forwards with a bind address of 127.0.0.1 vs "localhost" etc

we're waiting for a key exchange.

This should hopefully fix the situation where "~." doesn't work to terminate a
client session when a laptop wakes up. The client will be stuck waiting for a
key exchange on a dead connection, so won't have read the escape character

- Use _exit() in segv handler