hostinfo, tailcfg: add desktop detection on Linux to hostinfo

From the machines tab its hard to differenciate desktop Linux installs from
server Linux installs. Transmitting this information should make this
determination a lot easier.

Due to the reality that tailscaled is likely a system process, the standard
checks based on XDG_SESSION_TYPE or DISPLAY environment variables are not
possible (those variables won't be set). Instead, we look for listening
unix sockets that are typical of desktop installs.

Signed-off-by: Tom DNetto <tom@tailscale.com>

tailcfg/tailcfg_clone.go

@@ -117,6 +117,7 @@ var _HostinfoCloneNeedsRegeneration = Hostinfo(struct {
 	BackendLogID  string
 	OS            string
 	OSVersion     string
+	Desktop       opt.Bool
 	Package       string
 	DeviceModel   string
 	Hostname      string