MINOR: connection: add more connection error codes to cover common errno

MINOR: connection: add more connection error codes to cover common errno

While we get reports of connection setup errors in fc_err/bc_err, we
don't have the equivalent for the recv/send/splice syscalls. Let's
add provisions for new codes that cover the common errno values that
recv/send/splice can return, i.e. ECONNREFUSED, ENOMEM, EBADF, EFAULT,
EINVAL, ENOTCONN, ENOTSOCK, ENOBUFS, EPIPE. We also add a special case
for when the poller reported the error itself. It's worth noting that
EBADF/EFAULT/EINVAL will generally indicate serious bugs in the code
and should not be reported.

The only thing is that it's quite hard to forcefully (and reliably)
trigger these errors in automated tests as the timing is critical.
Using iptables to manually reset established connections in the
middle of large transfers at least permits to see some ECONNRESET
and/or EPIPE, but the other ones are harder to trigger.