Apple: reimplement DNS configuration reading

[bradh352]

The DNS configuration for apple is stored in the system configuration database. Apple does provide an emulated /etc/resolv.conf on MacOS (but not iOS), it cannot, however, represent the entirety of the DNS configuration. Alternatively, libresolv could be used to also retrieve some system configuration, but it too is not capable of retrieving the entirety of the DNS configuration.

Attempts to use the preferred public API of SCDynamicStoreCreate() and friends yielded incomplete DNS information. Instead, that leaves some apple "internal" symbols from configd that we need to access in order to get the entire configuration. We can see that we're not the only ones to do this as Google Chrome also does: https://chromium.googlesource.com/chromium/src/+/HEAD/net/dns/dns_config_watcher_mac.cc. These internal functions are what whatlibresolv and scutil use to retrieve the dns configuration. Since these symbols are not publicly available, we will dynamically load the symbols from libSystem and import the dnsinfo.h private header extracted from:
https://opensource.apple.com/source/configd/configd-1109.140.1/dnsinfo/dnsinfo.h.