When no custom resolver or host mapper is configured, xnet.Resolve now
resolves hostnames via Go's native pure-Go resolver (PreferGo: true)
instead of returning the hostname unchanged.
Previously, the unresolved hostname would reach net.Dialer.DialContext,
which on CGO_ENABLED=1 builds used cgo-based DNS — blocking a dedicated
OS thread per concurrent lookup. At scale (1000+ listeners), this
exhausted Go's 10000-thread limit.
Fixesgo-gost/gost#550