all: move nameserver resolution to public API

Make nameserver resolution functions more consistent and accessible:
- Rename currentNameserversFromResolvconf to CurrentNameserversFromResolvconf
- Move function to public API for better reusability
- Update all internal references to use the new public API
- Add comprehensive godoc comments for nameserver functions
- Improve code organization by centralizing DNS resolution logic

This change makes the nameserver resolution functionality more maintainable
and easier to use across different parts of the codebase.

resolver.go

@@ -647,7 +647,7 @@ func lookupIP(ctx context.Context, domain string, timeout int, bootstrapDNS []st
 // This is useful for doing PTR lookup in LAN network.
 func NewPrivateResolver(ctx context.Context) Resolver {
 	nss := initDefaultOsResolver(ctx)
-	resolveConfNss := currentNameserversFromResolvconf()
+	resolveConfNss := CurrentNameserversFromResolvconf()
 	localRfc1918Addrs := Rfc1918Addresses()
 	n := 0
 	for _, ns := range nss {