BUG/MEDIUM: dns: no DNS resolution happens if no ports provided to the nameserver

Erez reported a bug on discourse.haproxy.org about DNS resolution not
occuring when no port is specified on the nameserver directive.

This patch prevent this behavior by returning an error explaining this
issue when parsing the configuration file.
That said, later, we may want to force port 53 when client did not
provide any.

backport: 1.6
1 file changed