MEDIUM: resolvers: continue startup if network is unavailable

MEDIUM: resolvers: continue startup if network is unavailable

When haproxy starts with a resolver section, and there is a default one
since 2.6 which use /etc/resolv.conf, it tries to do a connect() with the UDP
socket in order to check if the routes of the system allows to reach the
server.

This check is too much restrictive as it won't prevent any runtime
failure.

Relax the check by making it a warning instead of a fatal alert.

This must be backported in 2.6.