Detect domain-specific resolvers on OS X and disable DnsClient

This CL also changes the handling of DNS configuration options not
yet fully implemented by DnsClient. Rather than DnsConfigService
failing to provide DnsConfig, the new field DnsConfig.unhandled_options
indicates to DnsClient that it should disable itself.


Review URL:

Cr-Mirrored-Commit: 3d164775c96e1e80601f28485483822182bfb666
2 files changed