net/dns: [win] add MagicDNS entries to etc/hosts

This works around the 2.3s delay in short name lookups when SNR is
enabled.
C:\Windows\System32\drivers\etc\hosts file. We only add known hosts that
match the search domains, and we populate the list in order of
Search Domains so that our matching algorithm mimics what Windows would
otherwise do itself if SNR was off.

Updates #1659

Signed-off-by: Maisem Ali <maisem@tailscale.com>

net/dns/osconfig.go

@@ -37,8 +37,18 @@ type OSConfigurator interface {
 	Close() error
 }
 
+// HostEntry represents a single line in the OS's hosts file.
+type HostEntry struct {
+	Addr  netip.Addr
+	Hosts []string
+}
+
 // OSConfig is an OS DNS configuration.
 type OSConfig struct {
+	// Hosts is a map of DNS FQDNs to their IPs, which should be added to the
+	// OS's hosts file. Currently, (2022-08-12) it is only populated for Windows
+	// in SplitDNS mode and with Smart Name Resolution turned on.
+	Hosts []*HostEntry
 	// Nameservers are the IP addresses of the nameservers to use.
 	Nameservers []netip.Addr
 	// SearchDomains are the domain suffixes to use when expanding