BUG/MINOR: config: Update cookie domain warn to RFC6265

BUG/MINOR: config: Update cookie domain warn to RFC6265

The domain option of the cookie keyword allows to define which domain or
domains should use the the cookie value of a cookie-based server
affinity. If the domain does not start with a dot, the user agent should
only use the cookie on hosts that matches the provided domains. If the
configured domain starts with a dot, the user agent can use the cookie
with any host ending with the configured domain.

haproxy config parser helps the admin warning about a potentially buggy
config: defining a domain without an embedded dot which does not start
with a dot, which is forbidden by the RFC.

The current condition to issue the warning implements RFC2109. This
change updates the implementation to RFC6265 which allows domain without
a leading dot.

Should be backported to all supported versions. The feature exists at least
since 1.5.