implement setup timeout

implement setup timeout

our current handling of only a specific set of error numerics makes us
vulnerable to hangs, when an unexpected code was received. we should
really respect a response timeout here (which must be quite large due to
ident!)

note that connect timeout is not handled by this (but by setsockopt);
it's a bit unfortunate that when that triggers, errno is EINPROGRESS
("Operation now in progress"), as that's a bit confusing.