[Fix] Fix rspamd nameserver round-robin when using /etc/resolv.conf

[Fix] Fix rspamd nameserver round-robin when using /etc/resolv.conf

When nameservers are parsed from /etc/resolv.conf, rspamd was setting
the upstream rotation strategy to RSPAMD_UPSTREAM_MASTER_SLAVE, which
caused it to only use the first nameserver unless it failed.

This behavior was inconsistent with the documented round-robin strategy
and with the behavior when nameservers are explicitly configured via
the configuration file.

Fixed by changing the rotation strategy to RSPAMD_UPSTREAM_ROUND_ROBIN
when parsing /etc/resolv.conf, matching the expected behavior.

Fixes RSP-284