BUG/MEDIUM: connection: always disable polling upon error

BUG/MEDIUM: connection: always disable polling upon error

Commit 0ffde2cc in 1.5-dev13 tried to always disable polling on file
descriptors when errors were encountered. Unfortunately it did not
always succeed in doing so because it relied on detecting polling
changes to disable it. Let's use a dedicated conn_stop_polling()
function that is inconditionally called upon error instead.

This managed to stop a busy loop observed when a health check makes
use of the send-proxy protocol and fails before the connection can
be established.